diff --git a/docs/en/02_Developer_Guides/00_Model/02_Relations.md b/docs/en/02_Developer_Guides/00_Model/02_Relations.md
index c6933e2a7..2a9f2da4f 100644
--- a/docs/en/02_Developer_Guides/00_Model/02_Relations.md
+++ b/docs/en/02_Developer_Guides/00_Model/02_Relations.md
@@ -420,6 +420,45 @@ If your object is versioned, cascade_deletes will also act as "cascade unpublish
 on a parent object will trigger unpublish on the child, similarly to how `owns` causes triggered publishing.
 See the [versioning docs](/developer_guides/versioning) for more information on ownership.
 
+## Cascading duplications
+
+Similar to `cascade_deletes` there is also a `cascade_duplicates` config which works in much the same way.
+When you invoke `$dataObject->duplicate()`, relation names specified by this config will be duplicated
+and saved against the new clone object.
+
+Note that duplications will act differently depending on the kind of relation:
+ - Exclusive relationships (e.g. has_many, belongs_to) will be explicitly duplicated.
+ - Non-exclusive many_many will not be duplicated, but the mapping table values will instead
+   be copied for this record.
+ - Non-exclusive has_one relationships are not normally necessary to duplicate, since both parent and clone
+   can normally share the same relation ID. However, if this is declared in `cascade_duplicates` any
+   has one will be similarly duplicated as though it were an exclusive relationship.
+
+For example:
+
+```php
+use SilverStripe\ORM\DataObject;
+
+class ParentObject extends DataObject {
+    private static $many_many = [
+        'RelatedChildren' => ChildObject::class,
+    ];
+    private static $cascade_duplicates = [ 'RelatedChildren' ];
+}
+class ChildObject extends DataObject {
+}
+```
+
+When duplicating objects you can disable recursive duplication by passing in `false` to the second
+argument of duplicate().
+
+E.g.
+
+```php
+$parent = ParentObject::get()->first();
+$dupe = $parent->duplicate(true, false);
+```
+
 ## Adding relations
 
 Adding new items to a relations works the same, regardless if you're editing a **has_many** or a **many_many**. They are 
diff --git a/src/Forms/FileUploadReceiver.php b/src/Forms/FileUploadReceiver.php
index 2a7839467..6a7c5688a 100644
--- a/src/Forms/FileUploadReceiver.php
+++ b/src/Forms/FileUploadReceiver.php
@@ -139,7 +139,13 @@ trait FileUploadReceiver
         $items = new ArrayList();
 
         // Determine format of presented data
-        if (empty($value) && $record) {
+        if ($value instanceof File) {
+            $items = ArrayList::create([$value]);
+            $value = null;
+        } elseif ($value instanceof SS_List) {
+            $items = $value;
+            $value = null;
+        } elseif (empty($value) && $record) {
             // If a record is given as a second parameter, but no submitted values,
             // then we should inspect this instead for the form values
 
@@ -158,7 +164,7 @@ trait FileUploadReceiver
                 // If directly passing a list then save the items directly
                 $items = $record;
             }
-        } elseif (!empty($value['Files'])) {
+        } elseif (is_array($value) && !empty($value['Files'])) {
             // If value is given as an array (such as a posted form), extract File IDs from this
             $class = $this->getRelationAutosetClass();
             $items = DataObject::get($class)->byIDs($value['Files']);
diff --git a/src/ORM/DataList.php b/src/ORM/DataList.php
index 570ecaef0..4766db9f8 100644
--- a/src/ORM/DataList.php
+++ b/src/ORM/DataList.php
@@ -1088,6 +1088,7 @@ class DataList extends ViewableData implements SS_List, Filterable, Sortable, Li
      * list manipulation
      *
      * @param mixed $item
+     * @param array|null $extraFields Any extra fields, if supported by this list
      */
     public function add($item)
     {
diff --git a/src/ORM/DataObject.php b/src/ORM/DataObject.php
index 420ad6dea..30d2c426d 100644
--- a/src/ORM/DataObject.php
+++ b/src/ORM/DataObject.php
@@ -259,7 +259,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
      *
      * @var DataObject[]
      */
-    protected $components;
+    protected $components = [];
 
     /**
      * Non-static cache of has_many and many_many relations that can't be written until this object is saved.
@@ -279,6 +279,18 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
      */
     private static $cascade_deletes = [];
 
+    /**
+     * List of relations that should be cascade duplicate.
+     * many_many duplications are shallow only.
+     *
+     * Note: If duplicating a many_many through you should refer to the
+     * has_many intermediary relation instead, otherwise extra fields
+     * will be omitted from the duplicated relation.
+     *
+     * @var array
+     */
+    private static $cascade_duplicates = [];
+
     /**
      * Get schema object
      *
@@ -292,7 +304,6 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
     /**
      * Construct a new DataObject.
      *
-
      * @param array|null $record Used internally for rehydrating an object from database content.
      *                           Bypasses setters on this class, and hence should not be used
      *                           for populating data on new records.
@@ -396,40 +407,105 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
      *
      * @param bool $doWrite Perform a write() operation before returning the object.
      * If this is true, it will create the duplicate in the database.
-     * @param bool|string $manyMany Which many-many to duplicate. Set to true to duplicate all, false to duplicate none.
-     * Alternatively set to the string of the relation config to duplicate
-     * (supports 'many_many', or 'belongs_many_many')
+     * @param array|null|false $relations List of relations to duplicate.
+     * Will default to `cascade_duplicates` if null.
+     * Set to 'false' to force none.
+     * Set to specific array of names to duplicate to override these.
+     * Note: If using versioned, this will additionally failover to `owns` config.
      * @return static A duplicate of this node. The exact type will be the type of this node.
      */
-    public function duplicate($doWrite = true, $manyMany = 'many_many')
+    public function duplicate($doWrite = true, $relations = null)
     {
+        // Handle legacy behaviour
+        if (is_string($relations) || $relations === true) {
+            if ($relations === true) {
+                $relations = 'many_many';
+            }
+            Deprecation::notice('5.0', 'Use cascade_duplicates config instead of providing a string to duplicate()');
+            $relations = array_keys($this->config()->get($relations)) ?: [];
+        }
+
+        // Get duplicates
+        if ($relations === null) {
+            $relations = $this->config()->get('cascade_duplicates');
+        }
+
+        // Create unsaved raw duplicate
         $map = $this->toMap();
         unset($map['Created']);
         /** @var static $clone */
         $clone = Injector::inst()->create(static::class, $map, false, $this->getSourceQueryParams());
         $clone->ID = 0;
 
-        $clone->invokeWithExtensions('onBeforeDuplicate', $this, $doWrite, $manyMany);
-        if ($manyMany) {
-            $this->duplicateManyManyRelations($this, $clone, $manyMany);
+        // Note: Extensions such as versioned may update $relations here
+        $clone->invokeWithExtensions('onBeforeDuplicate', $this, $doWrite, $relations);
+        if ($relations) {
+            $this->duplicateRelations($this, $clone, $relations);
         }
         if ($doWrite) {
             $clone->write();
         }
-        $clone->invokeWithExtensions('onAfterDuplicate', $this, $doWrite, $manyMany);
+        $clone->invokeWithExtensions('onAfterDuplicate', $this, $doWrite, $relations);
 
         return $clone;
     }
 
+    /**
+     * Copies the given relations from this object to the destination
+     *
+     * @param DataObject $sourceObject the source object to duplicate from
+     * @param DataObject $destinationObject the destination object to populate with the duplicated relations
+     * @param array $relations List of relations
+     */
+    protected function duplicateRelations($sourceObject, $destinationObject, $relations)
+    {
+        // Get list of duplicable relation types
+        $manyMany = $sourceObject->manyMany();
+        $hasMany = $sourceObject->hasMany();
+        $hasOne = $sourceObject->hasOne();
+        $belongsTo = $sourceObject->belongsTo();
+
+        // Duplicate each relation based on type
+        foreach ($relations as $relation) {
+            switch (true) {
+                case array_key_exists($relation, $manyMany): {
+                    $this->duplicateManyManyRelation($sourceObject, $destinationObject, $relation);
+                    break;
+                }
+                case array_key_exists($relation, $hasMany): {
+                    $this->duplicateHasManyRelation($sourceObject, $destinationObject, $relation);
+                    break;
+                }
+                case array_key_exists($relation, $hasOne): {
+                    $this->duplicateHasOneRelation($sourceObject, $destinationObject, $relation);
+                    break;
+                }
+                case array_key_exists($relation, $belongsTo): {
+                    $this->duplicateBelongsToRelation($sourceObject, $destinationObject, $relation);
+                    break;
+                }
+                default: {
+                    $sourceType = get_class($sourceObject);
+                    throw new InvalidArgumentException(
+                        "Cannot duplicate unknown relation {$relation} on parent type {$sourceType}"
+                    );
+                }
+            }
+        }
+    }
+
     /**
      * Copies the many_many and belongs_many_many relations from one object to another instance of the name of object.
      *
+     * @deprecated 4.1...5.0 Use duplicateRelations() instead
      * @param DataObject $sourceObject the source object to duplicate from
      * @param DataObject $destinationObject the destination object to populate with the duplicated relations
      * @param bool|string $filter
      */
     protected function duplicateManyManyRelations($sourceObject, $destinationObject, $filter)
     {
+        Deprecation::notice('5.0', 'Use duplicateRelations() instead');
+
         // Get list of relations to duplicate
         if ($filter === 'many_many' || $filter === 'belongs_many_many') {
             $relations = $sourceObject->config()->get($filter);
@@ -444,25 +520,93 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
     }
 
     /**
-     * Duplicates a single many_many relation from one object to another
+     * Duplicates a single many_many relation from one object to another.
      *
      * @param DataObject $sourceObject
      * @param DataObject $destinationObject
-     * @param string $manyManyName
+     * @param string $relation
      */
-    protected function duplicateManyManyRelation($sourceObject, $destinationObject, $manyManyName)
+    protected function duplicateManyManyRelation($sourceObject, $destinationObject, $relation)
     {
-        // Ensure this component exists on the destination side as well
-        if (!static::getSchema()->manyManyComponent(get_class($destinationObject), $manyManyName)) {
+        // Copy all components from source to destination
+        $source = $sourceObject->getManyManyComponents($relation);
+        $dest = $destinationObject->getManyManyComponents($relation);
+        $extraFieldNames = $source->getExtraFields();
+        foreach ($source as $item) {
+            // Merge extra fields
+            $extraFields = [];
+            foreach ($extraFieldNames as $fieldName => $fieldType) {
+                $extraFields[$fieldName] = $item->getField($fieldName);
+            }
+            $dest->add($item, $extraFields);
+        }
+    }
+
+    /**
+     * Duplicates a single many_many relation from one object to another.
+     *
+     * @param DataObject $sourceObject
+     * @param DataObject $destinationObject
+     * @param string $relation
+     */
+    protected function duplicateHasManyRelation($sourceObject, $destinationObject, $relation)
+    {
+        // Copy all components from source to destination
+        $source = $sourceObject->getComponents($relation);
+        $dest = $destinationObject->getComponents($relation);
+
+        /** @var DataObject $item */
+        foreach ($source as $item) {
+            // Don't write on duplicate; Wait until ParentID is available later.
+            // writeRelations() will eventually write these records when converting
+            // from UnsavedRelationList
+            $clonedItem = $item->duplicate(false);
+            $dest->add($clonedItem);
+        }
+    }
+
+    /**
+     * Duplicates a single has_one relation from one object to another.
+     * Note: Child object will be force written.
+     *
+     * @param DataObject $sourceObject
+     * @param DataObject $destinationObject
+     * @param string $relation
+     */
+    protected function duplicateHasOneRelation($sourceObject, $destinationObject, $relation)
+    {
+        // Check if original object exists
+        $item = $sourceObject->getComponent($relation);
+        if (!$item->isInDB()) {
             return;
         }
 
-        // Copy all components from source to destination
-        $source = $sourceObject->getManyManyComponents($manyManyName);
-        $dest = $destinationObject->getManyManyComponents($manyManyName);
-        foreach ($source as $item) {
-            $dest->add($item);
+        $clonedItem = $item->duplicate(false);
+        $destinationObject->setComponent($relation, $clonedItem);
+    }
+
+    /**
+     * Duplicates a single belongs_to relation from one object to another.
+     * Note: This will force a write on both parent / child objects.
+     *
+     * @param DataObject $sourceObject
+     * @param DataObject $destinationObject
+     * @param string $relation
+     */
+    protected function duplicateBelongsToRelation($sourceObject, $destinationObject, $relation)
+    {
+        // Check if original object exists
+        $item = $sourceObject->getComponent($relation);
+        if (!$item->isInDB()) {
+            return;
         }
+
+        $clonedItem = $item->duplicate(false);
+        $destinationObject->setComponent($relation, $clonedItem);
+        // After $clonedItem is assigned the appropriate FieldID / FieldClass, force write
+        // @todo Write this component in onAfterWrite instead, assigning the FieldID then
+        // https://github.com/silverstripe/silverstripe-framework/issues/7818
+        $clonedItem->write();
     }
 
     /**
@@ -561,7 +705,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         parent::defineMethods();
 
         if (static::class === self::class) {
-             return;
+            return;
         }
 
         // Set up accessors for joined items
@@ -639,7 +783,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         return i18n::_t(
             static::class . '.PLURALS',
             $default,
-            [ 'count' => $count ]
+            ['count' => $count]
         );
     }
 
@@ -808,7 +952,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
                         $parentObj = $relObj;
                         $relObj = $relObj->$relation();
                         // If the intermediate relationship objects haven't been created, then write them
-                        if ($i<sizeof($relations)-1 && !$relObj->ID || (!$relObj->ID && $parentObj !== $this)) {
+                        if ($i < sizeof($relations) - 1 && !$relObj->ID || (!$relObj->ID && $parentObj !== $this)) {
                             $relObj->write();
                             $relatedFieldName = $relation . "ID";
                             $parentObj->$relatedFieldName = $relObj->ID;
@@ -1122,11 +1266,11 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
 
             if ($defaults) {
                 foreach ($defaults as $fieldName => $fieldValue) {
-                // SRM 2007-03-06: Stricter check
+                    // SRM 2007-03-06: Stricter check
                     if (!isset($this->$fieldName) || $this->$fieldName === null) {
                         $this->$fieldName = $fieldValue;
                     }
-                // Set many-many defaults with an array of ids
+                    // Set many-many defaults with an array of ids
                     if (is_array($fieldValue) && $this->getSchema()->manyManyComponent(static::class, $fieldName)) {
                         /** @var ManyManyList $manyManyJoin */
                         $manyManyJoin = $this->$fieldName();
@@ -1232,7 +1376,11 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
             }
 
             // Ensure this field pertains to this table
-            $specification = $schema->fieldSpec($class, $fieldName, DataObjectSchema::DB_ONLY | DataObjectSchema::UNINHERITED);
+            $specification = $schema->fieldSpec(
+                $class,
+                $fieldName,
+                DataObjectSchema::DB_ONLY | DataObjectSchema::UNINHERITED
+            );
             if (!$specification) {
                 continue;
             }
@@ -1251,10 +1399,9 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         if ($baseTable === $table) {
             $manipulation[$table]['fields']['LastEdited'] = $now;
             if ($isNewRecord) {
-                $manipulation[$table]['fields']['Created']
-                    = empty($this->record['Created'])
-                        ? $now
-                        : $this->record['Created'];
+                $manipulation[$table]['fields']['Created'] = empty($this->record['Created'])
+                    ? $now
+                    : $this->record['Created'];
                 $manipulation[$table]['fields']['ClassName'] = static::class;
             }
         }
@@ -1325,7 +1472,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
      *  - $this->onBeforeWrite() gets called beforehand.
      *  - Extensions such as Versioned will ammend the database-write to ensure that a version is saved.
      *
-     *  @uses DataExtension->augmentWrite()
+     * @uses DataExtension->augmentWrite()
      *
      * @param boolean $showDebug Show debugging information
      * @param boolean $forceInsert Run INSERT command rather than UPDATE, even if record already exists
@@ -1414,10 +1561,8 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
      */
     public function writeComponents($recursive = false)
     {
-        if ($this->components) {
-            foreach ($this->components as $component) {
-                $component->write(false, false, false, $recursive);
-            }
+        foreach ($this->components as $component) {
+            $component->write(false, false, false, $recursive);
         }
 
         if ($join = $this->getJoin()) {
@@ -1431,7 +1576,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
      * Delete this data object.
      * $this->onBeforeDelete() gets called.
      * Note that in Versioned objects, both Stage and Live will be deleted.
-     *  @uses DataExtension->augmentSQL()
+     * @uses DataExtension->augmentSQL()
      */
     public function delete()
     {
@@ -1501,7 +1646,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
     }
 
     /**
-     * Return a component object from a one to one relationship, as a DataObject.
+     * Return a unary component object from a one to one relationship, as a DataObject.
      * If no component is available, an 'empty component' will be returned for
      * non-polymorphic relations, or for polymorphic relations with a class set.
      *
@@ -1518,7 +1663,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         $schema = static::getSchema();
         if ($class = $schema->hasOneComponent(static::class, $componentName)) {
             $joinField = $componentName . 'ID';
-            $joinID    = $this->getField($joinField);
+            $joinID = $this->getField($joinField);
 
             // Extract class name for polymorphic relations
             if ($class === self::class) {
@@ -1582,6 +1727,52 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         return $component;
     }
 
+    /**
+     * Assign an item to the given component
+     *
+     * @param string $componentName
+     * @param DataObject|null $item
+     * @return $this
+     */
+    public function setComponent($componentName, $item)
+    {
+        // Validate component
+        $schema = static::getSchema();
+        if ($class = $schema->hasOneComponent(static::class, $componentName)) {
+            // Force item to be written if not by this point
+            // @todo This could be lazy-written in a beforeWrite hook, but force write here for simplicity
+            // https://github.com/silverstripe/silverstripe-framework/issues/7818
+            if ($item && !$item->isInDB()) {
+                $item->write();
+            }
+
+            // Update local ID
+            $joinField = $componentName . 'ID';
+            $this->setField($joinField, $item ? $item->ID : null);
+            // Update Class (Polymorphic has_one)
+            // Extract class name for polymorphic relations
+            if ($class === self::class) {
+                $this->setField($componentName . 'Class', $item ? get_class($item) : null);
+            }
+        } elseif ($class = $schema->belongsToComponent(static::class, $componentName)) {
+            if ($item) {
+                // For belongs_to, add to has_one on other component
+                $joinField = $schema->getRemoteJoinField(static::class, $componentName, 'belongs_to', $polymorphic);
+                if (!$polymorphic) {
+                    $joinField = substr($joinField, 0, -2);
+                }
+                $item->setComponent($joinField, $this);
+            }
+        } else {
+            throw new InvalidArgumentException(
+                "DataObject->setComponent(): Could not find component '$componentName'."
+            );
+        }
+
+        $this->components[$componentName] = $item;
+        return $this;
+    }
+
     /**
      * Returns a one-to-many relation as a HasManyList
      *
@@ -1655,7 +1846,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
             $remoteClass = $candidates[$relationName];
 
             // If dot notation is present, extract just the first part that contains the class.
-            if (($fieldPos = strpos($remoteClass, '.'))!==false) {
+            if (($fieldPos = strpos($remoteClass, '.')) !== false) {
                 return substr($remoteClass, 0, $fieldPos);
             }
 
@@ -1752,7 +1943,12 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
             case 'belongs_to':
             case 'has_many': {
                 // These relations must have a has_one on the other end, so find it
-                $joinField = $schema->getRemoteJoinField($remoteClass, $remoteRelation, $relationType, $polymorphic);
+                $joinField = $schema->getRemoteJoinField(
+                    $remoteClass,
+                    $remoteRelation,
+                    $relationType,
+                    $polymorphic
+                );
                 if ($polymorphic) {
                     throw new InvalidArgumentException(sprintf(
                         "%s cannot generate opposite component of relation %s.%s, as the other end appears" .
@@ -1806,7 +2002,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
      * Returns a many-to-many component, as a ManyManyList.
      * @param string $componentName Name of the many-many component
      * @param int|array $id Optional ID for parent of this relation, if not the current record
-     * @return RelationList|UnsavedRelationList The set of components
+     * @return ManyManyList|UnsavedRelationList The set of components
      */
     public function getManyManyComponents($componentName, $id = null)
     {
@@ -1827,7 +2023,11 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         if (!$id) {
             if (!isset($this->unsavedRelations[$componentName])) {
                 $this->unsavedRelations[$componentName] =
-                    new UnsavedRelationList($manyManyComponent['parentClass'], $componentName, $manyManyComponent['childClass']);
+                    new UnsavedRelationList(
+                        $manyManyComponent['parentClass'],
+                        $componentName,
+                        $manyManyComponent['childClass']
+                    );
             }
             return $this->unsavedRelations[$componentName];
         }
@@ -2180,9 +2380,15 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
             $tableClass = $this->record[$field . '_Lazy'];
             $this->loadLazyFields($tableClass);
         }
+        $schema = static::getSchema();
+
+        // Support unary relations as fields
+        if ($schema->unaryComponent(static::class, $field)) {
+            return $this->getComponent($field);
+        }
 
         // In case of complex fields, return the DBField object
-        if (static::getSchema()->compositeField(static::class, $field)) {
+        if ($schema->compositeField(static::class, $field)) {
             $this->record[$field] = $this->dbObject($field);
         }
 
@@ -2331,9 +2537,9 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         if ($fields) {
             foreach ($fields as $name => $level) {
                 $changedFields[$name] = array(
-                'before' => array_key_exists($name, $this->original) ? $this->original[$name] : null,
-                'after' => array_key_exists($name, $this->record) ? $this->record[$name] : null,
-                'level' => $level
+                    'before' => array_key_exists($name, $this->original) ? $this->original[$name] : null,
+                    'after' => array_key_exists($name, $this->record) ? $this->record[$name] : null,
+                    'level' => $level
                 );
             }
         }
@@ -2382,6 +2588,18 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
             $this->loadLazyFields($tableClass);
         }
 
+        // Support component assignent via field setter
+        $schema = static::getSchema();
+        if ($schema->unaryComponent(static::class, $fieldName)) {
+            // Assign component directly
+            if (is_null($val) || $val instanceof DataObject) {
+                return $this->setComponent($fieldName, $val);
+            }
+            // Assign by ID instead of object
+            unset($this->components[$fieldName]);
+            $fieldName .= 'ID';
+        }
+
         // Situation 1: Passing an DBField
         if ($val instanceof DBField) {
             $val->setName($fieldName);
@@ -2396,7 +2614,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         // Situation 2: Passing a literal or non-DBField object
         } else {
             // If this is a proper database field, we shouldn't be getting non-DBField objects
-            if (is_object($val) && static::getSchema()->fieldSpec(static::class, $fieldName)) {
+            if (is_object($val) && $schema->fieldSpec(static::class, $fieldName)) {
                 throw new InvalidArgumentException('DataObject::setField: passed an object that is not a DBField');
             }
 
@@ -2483,8 +2701,9 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         $schema = static::getSchema();
         return (
             array_key_exists($field, $this->record)
+            || array_key_exists($field, $this->components)
             || $schema->fieldSpec(static::class, $field)
-            || (substr($field, -2) == 'ID') && $schema->hasOneComponent(static::class, substr($field, 0, -2))
+            || $schema->unaryComponent(static::class, $field)
             || $this->hasMethod("get{$field}")
         );
     }
@@ -2717,7 +2936,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
 
             // Inspect relation type
             if (ClassInfo::hasMethod($component, $relation)) {
-                        $component = $component->$relation();
+                $component = $component->$relation();
             } elseif ($component instanceof Relation || $component instanceof DataList) {
                 // $relation could either be a field (aggregate), or another relation
                 $singleton = DataObject::singleton($component->dataClass());
@@ -3120,7 +3339,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
                     ],
                     $childField => [
                         'type' => 'index',
-                        'name' =>$childField,
+                        'name' => $childField,
                         'columns' => [$childField],
                     ],
                 ];
@@ -3174,7 +3393,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
         $labels = $this->fieldLabels();
 
         // fallback to summary fields (unless empty array is explicitly specified)
-        if (! $fields && ! is_array($fields)) {
+        if (!$fields && !is_array($fields)) {
             $summaryFields = array_keys($this->summaryFields());
             $fields = array();
 
@@ -3288,13 +3507,28 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
                     ];
                     if ($includerelations) {
                         $types['has_one'] = (array)Config::inst()->get($ancestorClass, 'has_one', Config::UNINHERITED);
-                        $types['has_many'] = (array)Config::inst()->get($ancestorClass, 'has_many', Config::UNINHERITED);
-                        $types['many_many'] = (array)Config::inst()->get($ancestorClass, 'many_many', Config::UNINHERITED);
-                        $types['belongs_many_many'] = (array)Config::inst()->get($ancestorClass, 'belongs_many_many', Config::UNINHERITED);
+                        $types['has_many'] = (array)Config::inst()->get(
+                            $ancestorClass,
+                            'has_many',
+                            Config::UNINHERITED
+                        );
+                        $types['many_many'] = (array)Config::inst()->get(
+                            $ancestorClass,
+                            'many_many',
+                            Config::UNINHERITED
+                        );
+                        $types['belongs_many_many'] = (array)Config::inst()->get(
+                            $ancestorClass,
+                            'belongs_many_many',
+                            Config::UNINHERITED
+                        );
                     }
                     foreach ($types as $type => $attrs) {
                         foreach ($attrs as $name => $spec) {
-                            $autoLabels[$name] = _t("{$ancestorClass}.{$type}_{$name}", FormField::name_to_label($name));
+                            $autoLabels[$name] = _t(
+                                "{$ancestorClass}.{$type}_{$name}",
+                                FormField::name_to_label($name)
+                            );
                         }
                     }
                 }
@@ -3428,6 +3662,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
     {
         self::$subclass_access = false;
     }
+
     public static function enable_subclass_access()
     {
         self::$subclass_access = true;
@@ -3523,7 +3758,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
      *
      * Note that you cannot have a has_one and belongs_to relationship with the same name.
      *
-     *  @var array
+     * @var array
      * @config
      */
     private static $has_one = [];
diff --git a/src/ORM/DataObjectSchema.php b/src/ORM/DataObjectSchema.php
index 01db82927..e309a1660 100644
--- a/src/ORM/DataObjectSchema.php
+++ b/src/ORM/DataObjectSchema.php
@@ -908,6 +908,19 @@ class DataObjectSchema
         return $classOnly ? $belongsToClass : $belongsTo;
     }
 
+    /**
+     * Check class for any unary component
+     *
+     * Alias for hasOneComponent() ?: belongsToComponent()
+     * @param string $class
+     * @param string $component
+     * @return string|null
+     */
+    public function unaryComponent($class, $component)
+    {
+        return $this->hasOneComponent($class, $component) ?: $this->belongsToComponent($class, $component);
+    }
+
     /**
      *
      * @param string $parentClass Parent class name
diff --git a/src/ORM/ManyManyList.php b/src/ORM/ManyManyList.php
index c259890fa..68fee0d91 100644
--- a/src/ORM/ManyManyList.php
+++ b/src/ORM/ManyManyList.php
@@ -225,6 +225,10 @@ class ManyManyList extends RelationList
         if (is_numeric($item)) {
             $itemID = $item;
         } elseif ($item instanceof $this->dataClass) {
+            // Ensure record is saved
+            if (!$item->isInDB()) {
+                $item->write();
+            }
             $itemID = $item->ID;
         } else {
             throw new InvalidArgumentException(
@@ -232,7 +236,7 @@ class ManyManyList extends RelationList
             );
         }
         if (empty($itemID)) {
-            throw new InvalidArgumentException("ManyManyList::add() doesn't accept unsaved records");
+            throw new InvalidArgumentException("ManyManyList::add() couldn't add this record");
         }
 
         // Validate foreignID
diff --git a/src/ORM/UnsavedRelationList.php b/src/ORM/UnsavedRelationList.php
index ea11dddbd..2885c89fc 100644
--- a/src/ORM/UnsavedRelationList.php
+++ b/src/ORM/UnsavedRelationList.php
@@ -82,10 +82,6 @@ class UnsavedRelationList extends ArrayList implements Relation
     public function changeToList(RelationList $list)
     {
         foreach ($this->items as $key => $item) {
-            if (is_object($item)) {
-                /** @var DataObject $item */
-                $item->write();
-            }
             $list->add($item, $this->extraFields[$key]);
         }
     }
diff --git a/tests/php/ORM/DataObjectDuplicationTest.php b/tests/php/ORM/DataObjectDuplicationTest.php
index b3186d53f..48b3fbf38 100644
--- a/tests/php/ORM/DataObjectDuplicationTest.php
+++ b/tests/php/ORM/DataObjectDuplicationTest.php
@@ -2,32 +2,30 @@
 
 namespace SilverStripe\ORM\Tests;
 
-use SilverStripe\ORM\DataObject;
 use SilverStripe\Dev\SapphireTest;
-use SilverStripe\ORM\FieldType\DBDatetime;
 
 class DataObjectDuplicationTest extends SapphireTest
 {
+    protected static $fixture_file = 'DataObjectDuplicationTest.yml';
 
-    protected $usesDatabase = true;
-
-    protected static $extra_dataobjects = array(
-        DataObjectDuplicationTest\Class1::class,
-        DataObjectDuplicationTest\Class2::class,
-        DataObjectDuplicationTest\Class3::class,
-        DataObjectDuplicationTest\Class4::class,
-    );
+    protected static $extra_dataobjects = [
+        DataObjectDuplicationTest\Antelope::class,
+        DataObjectDuplicationTest\Bobcat::class,
+        DataObjectDuplicationTest\Caribou::class,
+        DataObjectDuplicationTest\Dingo::class,
+        DataObjectDuplicationTest\Elephant::class,
+        DataObjectDuplicationTest\Frog::class,
+        DataObjectDuplicationTest\Goat::class,
+    ];
 
     public function testDuplicate()
     {
-        DBDatetime::set_mock_now('2016-01-01 01:01:01');
-        $orig = new DataObjectDuplicationTest\Class1();
-        $orig->text = 'foo';
-        $orig->write();
-        DBDatetime::set_mock_now('2016-01-02 01:01:01');
+        /** @var DataObjectDuplicationTest\Antelope $orig */
+        $orig = $this->objFromFixture(DataObjectDuplicationTest\Antelope::class, 'one');
+        /** @var DataObjectDuplicationTest\Antelope $duplicate */
         $duplicate = $orig->duplicate();
         $this->assertInstanceOf(
-            DataObjectDuplicationTest\Class1::class,
+            DataObjectDuplicationTest\Antelope::class,
             $duplicate,
             'Creates the correct type'
         );
@@ -36,198 +34,79 @@ class DataObjectDuplicationTest extends SapphireTest
             $orig->ID,
             'Creates a unique record'
         );
-        $this->assertEquals(
-            'foo',
-            $duplicate->text,
-            'Copies fields'
-        );
-        $this->assertEquals(
-            2,
-            DataObjectDuplicationTest\Class1::get()->Count(),
-            'Only creates a single duplicate'
-        );
-        $this->assertEquals(DBDatetime::now()->Nice(), $duplicate->dbObject('Created')->Nice());
-        $this->assertNotEquals($orig->dbObject('Created')->Nice(), $duplicate->dbObject('Created')->Nice());
-    }
 
-    public function testDuplicateHasOne()
-    {
-        $relationObj = new DataObjectDuplicationTest\Class1();
-        $relationObj->text = 'class1';
-        $relationObj->write();
-
-        $orig = new DataObjectDuplicationTest\Class2();
-        $orig->text = 'class2';
-        $orig->oneID = $relationObj->ID;
-        $orig->write();
-
-        $duplicate = $orig->duplicate();
-        $this->assertEquals(
-            $relationObj->ID,
-            $duplicate->oneID,
-            'Copies has_one relationship'
-        );
-        $this->assertEquals(
-            2,
-            DataObjectDuplicationTest\Class2::get()->Count(),
-            'Only creates a single duplicate'
-        );
-        $this->assertEquals(
-            1,
-            DataObjectDuplicationTest\Class1::get()->Count(),
-            'Does not create duplicate of has_one relationship'
-        );
-    }
-
-
-    public function testDuplicateManyManyClasses()
-    {
-        //create new test classes below
-        $one = new DataObjectDuplicationTest\Class1();
-        $two = new DataObjectDuplicationTest\Class2();
-        $three = new DataObjectDuplicationTest\Class3();
-
-        //set some simple fields
-        $text1 = "Test Text 1";
-        $text2 = "Test Text 2";
-        $text3 = "Test Text 3";
-        $one->text = $text1;
-        $two->text = $text2;
-        $three->text = $text3;
-
-        //write the to DB
-        $one->write();
-        $two->write();
-        $three->write();
-
-        //create relations
-        $one->twos()->add($two);
-        $one->threes()->add($three);
-
-        $one = DataObject::get_by_id(DataObjectDuplicationTest\Class1::class, $one->ID);
-        $two = DataObject::get_by_id(DataObjectDuplicationTest\Class2::class, $two->ID);
-        $three = DataObject::get_by_id(DataObjectDuplicationTest\Class3::class, $three->ID);
-
-        //test duplication
-        $oneCopy = $one->duplicate(true, true);
-        $twoCopy = $two->duplicate(true, true);
-        $threeCopy = $three->duplicate(true, true);
-
-        $oneCopy = DataObject::get_by_id(DataObjectDuplicationTest\Class1::class, $oneCopy->ID);
-        $twoCopy = DataObject::get_by_id(DataObjectDuplicationTest\Class2::class, $twoCopy->ID);
-        $threeCopy = DataObject::get_by_id(DataObjectDuplicationTest\Class3::class, $threeCopy->ID);
-
-        $this->assertNotNull($oneCopy, "Copy of 1 exists");
-        $this->assertNotNull($twoCopy, "Copy of 2 exists");
-        $this->assertNotNull($threeCopy, "Copy of 3 exists");
-
-        $this->assertEquals($text1, $oneCopy->text);
-        $this->assertEquals($text2, $twoCopy->text);
-        $this->assertEquals($text3, $threeCopy->text);
-
-        $this->assertNotEquals(
-            $one->twos()->Count(),
-            $oneCopy->twos()->Count(),
-            "Many-to-one relation not copied (has_many)"
-        );
-        $this->assertEquals(
-            $one->threes()->Count(),
-            $oneCopy->threes()->Count(),
-            "Object has the correct number of relations"
-        );
-        $this->assertEquals(
-            $three->ones()->Count(),
-            $threeCopy->ones()->Count(),
-            "Object has the correct number of relations"
-        );
-
-        $this->assertEquals(
-            $one->ID,
-            $twoCopy->one()->ID,
-            "Match between relation of copy and the original"
-        );
-        $this->assertEquals(
-            0,
-            $oneCopy->twos()->Count(),
-            "Many-to-one relation not copied (has_many)"
-        );
-        $this->assertEquals(
-            $three->ID,
-            $oneCopy->threes()->First()->ID,
-            "Match between relation of copy and the original"
-        );
-        $this->assertEquals(
-            $one->ID,
-            $threeCopy->ones()->First()->ID,
-            "Match between relation of copy and the original"
-        );
-    }
-
-    public function testDuplicateManyManyFiltered()
-    {
-        $parent = new DataObjectDuplicationTest\Class4();
-        $parent->Title = 'Parent';
-        $parent->write();
-
-        $child = new DataObjectDuplicationTest\Class4();
-        $child->Title = 'Child';
-        $child->write();
-
-        $grandChild = new DataObjectDuplicationTest\Class4();
-        $grandChild->Title = 'GrandChild';
-        $grandChild->write();
-
-        $parent->Children()->add($child);
-        $child->Children()->add($grandChild);
-
-        // Duplcating $child should only duplicate grandchild
-        $childDuplicate = $child->duplicate(true, 'many_many');
-        $this->assertEquals(0, $childDuplicate->Parents()->count());
+        // Check 'bobcats' relation duplicated
+        $twoOne = $this->objFromFixture(DataObjectDuplicationTest\Bobcat::class, 'one');
+        $twoTwo = $this->objFromFixture(DataObjectDuplicationTest\Bobcat::class, 'two');
         $this->assertListEquals(
-            [['Title' => 'GrandChild']],
-            $childDuplicate->Children()
+            [
+                ['Title' => 'Bobcat two'],
+                ['Title' => 'Bobcat three'],
+            ],
+            $duplicate->bobcats()
         );
-
-        // Duplicate belongs_many_many only
-        $belongsDuplicate = $child->duplicate(true, 'belongs_many_many');
-        $this->assertEquals(0, $belongsDuplicate->Children()->count());
-        $this->assertListEquals(
-            [['Title' => 'Parent']],
-            $belongsDuplicate->Parents()
+        $this->assertEmpty(
+            array_intersect(
+                $orig->bobcats()->getIDList(),
+                $duplicate->bobcats()->getIDList()
+            )
         );
+        /** @var DataObjectDuplicationTest\Bobcat $twoTwoDuplicate */
+        $twoTwoDuplicate = $duplicate->bobcats()->filter('Title', 'Bobcat two')->first();
+        $this->assertNotEmpty($twoTwoDuplicate);
+        $this->assertNotEquals($twoTwo->ID, $twoTwoDuplicate->ID);
 
-        // Duplicate all
-        $allDuplicate = $child->duplicate(true, true);
+        // Check 'bobcats.self' relation duplicated
+        /** @var DataObjectDuplicationTest\Bobcat $twoOneDuplicate */
+        $twoOneDuplicate = $twoTwoDuplicate->self();
+        $this->assertNotEmpty($twoOneDuplicate);
+        $this->assertNotEquals($twoOne->ID, $twoOneDuplicate->ID);
+
+        // Ensure 'bobcats.seven' instance is not duplicated
+        $sevenOne = $this->objFromFixture(DataObjectDuplicationTest\Goat::class, 'one');
+        $sevenTwo = $this->objFromFixture(DataObjectDuplicationTest\Goat::class, 'two');
+        $this->assertEquals($sevenOne->ID, $twoOneDuplicate->goat()->ID);
+        $this->assertEquals($sevenTwo->ID, $twoTwoDuplicate->goat()->ID);
+
+        // Ensure that 'caribou' many_many list exists on both, but only the mapping table is duplicated
+        // many_many_extraFields are also duplicated
+        $caribouList = [
+            [
+                'Title' => 'Caribou one',
+                'Sort' => 4,
+            ],
+            [
+                'Title' => 'Caribou two',
+                'Sort' => 5,
+            ],
+        ];
+        // Original and duplicate lists have the same content
         $this->assertListEquals(
-            [['Title' => 'Parent']],
-            $allDuplicate->Parents()
+            $caribouList,
+            $orig->caribou()
         );
         $this->assertListEquals(
-            [['Title' => 'GrandChild']],
-            $allDuplicate->Children()
+            $caribouList,
+            $duplicate->caribou()
+        );
+        // Ids of each record are the same (only mapping content is duplicated)
+        $this->assertEquals(
+            $orig->caribou()->getIDList(),
+            $duplicate->caribou()->getIDList()
         );
-    }
 
-    /**
-     * Test duplication of UnsavedRelations
-     */
-    public function testDuplicateUnsaved()
-    {
-        $one = new DataObjectDuplicationTest\Class1();
-        $one->text = "Test Text 1";
-        $three = new DataObjectDuplicationTest\Class3();
-        $three->text = "Test Text 3";
-        $one->threes()->add($three);
-        $this->assertListEquals(
-            [['text' => 'Test Text 3']],
-            $one->threes()
-        );
-        // Test duplicate
-        $dupe = $one->duplicate(false, true);
-        $this->assertEquals('Test Text 1', $dupe->text);
-        $this->assertListEquals(
-            [['text' => 'Test Text 3']],
-            $dupe->threes()
-        );
+        // Ensure 'five' belongs_to is duplicated
+        $fiveOne = $this->objFromFixture(DataObjectDuplicationTest\Elephant::class, 'one');
+        $fiveOneDuplicate = $duplicate->elephant();
+        $this->assertNotEmpty($fiveOneDuplicate);
+        $this->assertEquals('Elephant one', $fiveOneDuplicate->Title);
+        $this->assertNotEquals($fiveOne->ID, $fiveOneDuplicate->ID);
+
+        // Ensure 'five.Child' is duplicated
+        $sixOne = $this->objFromFixture(DataObjectDuplicationTest\Frog::class, 'one');
+        $sixOneDuplicate = $fiveOneDuplicate->Child();
+        $this->assertNotEmpty($sixOneDuplicate);
+        $this->assertEquals('Frog one', $sixOneDuplicate->Title);
+        $this->assertNotEquals($sixOne->ID, $sixOneDuplicate->ID);
     }
 }
diff --git a/tests/php/ORM/DataObjectDuplicationTest.yml b/tests/php/ORM/DataObjectDuplicationTest.yml
new file mode 100644
index 000000000..8ab748d9f
--- /dev/null
+++ b/tests/php/ORM/DataObjectDuplicationTest.yml
@@ -0,0 +1,42 @@
+SilverStripe\ORM\Tests\DataObjectDuplicationTest\Antelope:
+  one:
+    Title: 'Antelope one'
+SilverStripe\ORM\Tests\DataObjectDuplicationTest\Goat:
+  one:
+    Title: 'Goat one'
+  two:
+    Title: 'Goat two'
+SilverStripe\ORM\Tests\DataObjectDuplicationTest\Bobcat:
+  one:
+    Title: 'Bobcat one'
+    goat: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Goat.one
+  two:
+    Title: 'Bobcat two'
+    antelope: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Antelope.one
+    self: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Bobcat.one
+    goat: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Goat.two
+  three:
+    Title: 'Bobcat three'
+    antelope: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Antelope.one
+SilverStripe\ORM\Tests\DataObjectDuplicationTest\Caribou:
+  one:
+    Title: 'Caribou one'
+  two:
+    Title: 'Caribou two'
+DataObjectDuplicateTest_Antelope_caribou:
+  one:
+    DataObjectDuplicateTest_AntelopeID: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Antelope.one
+    DataObjectDuplicateTest_CaribouID: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Caribou.one
+    Sort: 4
+  two:
+    DataObjectDuplicateTest_AntelopeID: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Antelope.one
+    DataObjectDuplicateTest_CaribouID: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Caribou.two
+    Sort: 5
+SilverStripe\ORM\Tests\DataObjectDuplicationTest\Frog:
+  one:
+    Title: 'Frog one'
+SilverStripe\ORM\Tests\DataObjectDuplicationTest\Elephant:
+  one:
+    Title: 'Elephant one'
+    Parent: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Antelope.one
+    Child: =>SilverStripe\ORM\Tests\DataObjectDuplicationTest\Frog.one
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Antelope.php b/tests/php/ORM/DataObjectDuplicationTest/Antelope.php
new file mode 100644
index 000000000..5507168ec
--- /dev/null
+++ b/tests/php/ORM/DataObjectDuplicationTest/Antelope.php
@@ -0,0 +1,46 @@
+<?php
+
+namespace SilverStripe\ORM\Tests\DataObjectDuplicationTest;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+use SilverStripe\ORM\HasManyList;
+use SilverStripe\ORM\ManyManyList;
+
+/**
+ * @method HasManyList bobcats()
+ * @method ManyManyList caribou()
+ * @method Elephant elephant()
+ */
+class Antelope extends DataObject implements TestOnly
+{
+    private static $table_name = 'DataObjectDuplicateTest_Antelope';
+
+    private static $cascade_duplicates = [
+        'bobcats',
+        'caribou',
+        'elephant',
+    ];
+
+    private static $db = [
+        'Title' => 'Varchar',
+    ];
+
+    private static $has_many = [
+        'bobcats' => Bobcat::class,
+    ];
+
+    private static $many_many = [
+        'caribou' => Caribou::class,
+    ];
+
+    private static $many_many_extraFields = [
+        'caribou' => [
+            'Sort' => 'Int',
+        ],
+    ];
+
+    private static $belongs_to = [
+        'elephant' => Elephant::class,
+    ];
+}
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Bobcat.php b/tests/php/ORM/DataObjectDuplicationTest/Bobcat.php
new file mode 100644
index 000000000..53c8e4ea2
--- /dev/null
+++ b/tests/php/ORM/DataObjectDuplicationTest/Bobcat.php
@@ -0,0 +1,30 @@
+<?php
+
+namespace SilverStripe\ORM\Tests\DataObjectDuplicationTest;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+
+/**
+ * @method Antelope antelope()
+ * @method Bobcat self()
+ * @method Goat goat()
+ */
+class Bobcat extends DataObject implements TestOnly
+{
+    private static $table_name = 'DataObjectDuplicateTest_Bobcat';
+
+    private static $cascade_duplicates = [
+        'self',
+    ];
+
+    private static $db = [
+        'Title' => 'Varchar',
+    ];
+
+    private static $has_one = [
+        'antelope' => Antelope::class,
+        'self' => Bobcat::class,
+        'goat' => Goat::class,
+    ];
+}
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Caribou.php b/tests/php/ORM/DataObjectDuplicationTest/Caribou.php
new file mode 100644
index 000000000..85f279678
--- /dev/null
+++ b/tests/php/ORM/DataObjectDuplicationTest/Caribou.php
@@ -0,0 +1,23 @@
+<?php
+
+namespace SilverStripe\ORM\Tests\DataObjectDuplicationTest;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+use SilverStripe\ORM\ManyManyList;
+
+/**
+ * @method ManyManyList antelopes()
+ */
+class Caribou extends DataObject implements TestOnly
+{
+    private static $table_name = 'DataObjectDuplicateTest_Caribou';
+
+    private static $db = [
+        'Title' => 'Varchar',
+    ];
+
+    private static $belongs_many_many = [
+        'antelopes' => Antelope::class,
+    ];
+}
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Class1.php b/tests/php/ORM/DataObjectDuplicationTest/Class1.php
deleted file mode 100644
index 57a6c8302..000000000
--- a/tests/php/ORM/DataObjectDuplicationTest/Class1.php
+++ /dev/null
@@ -1,23 +0,0 @@
-<?php
-
-namespace SilverStripe\ORM\Tests\DataObjectDuplicationTest;
-
-use SilverStripe\Dev\TestOnly;
-use SilverStripe\ORM\DataObject;
-
-class Class1 extends DataObject implements TestOnly
-{
-    private static $table_name = 'DataObjectDuplicateTest_Class1';
-
-    private static $db = array(
-        'text' => 'Varchar'
-    );
-
-    private static $has_many = array(
-        'twos' => Class2::class
-    );
-
-    private static $many_many = array(
-        'threes' => Class3::class
-    );
-}
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Class2.php b/tests/php/ORM/DataObjectDuplicationTest/Class2.php
deleted file mode 100644
index b1b96bf61..000000000
--- a/tests/php/ORM/DataObjectDuplicationTest/Class2.php
+++ /dev/null
@@ -1,19 +0,0 @@
-<?php
-
-namespace SilverStripe\ORM\Tests\DataObjectDuplicationTest;
-
-use SilverStripe\Dev\TestOnly;
-use SilverStripe\ORM\DataObject;
-
-class Class2 extends DataObject implements TestOnly
-{
-    private static $table_name = 'DataObjectDuplicateTest_Class2';
-
-    private static $db = array(
-        'text' => 'Varchar'
-    );
-
-    private static $has_one = array(
-        'one' => Class1::class
-    );
-}
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Class3.php b/tests/php/ORM/DataObjectDuplicationTest/Class3.php
deleted file mode 100644
index c018152da..000000000
--- a/tests/php/ORM/DataObjectDuplicationTest/Class3.php
+++ /dev/null
@@ -1,19 +0,0 @@
-<?php
-
-namespace SilverStripe\ORM\Tests\DataObjectDuplicationTest;
-
-use SilverStripe\Dev\TestOnly;
-use SilverStripe\ORM\DataObject;
-
-class Class3 extends DataObject implements TestOnly
-{
-    private static $table_name = 'DataObjectDuplicateTest_Class3';
-
-    private static $db = array(
-        'text' => 'Varchar'
-    );
-
-    private static $belongs_many_many = array(
-        'ones' => Class1::class
-    );
-}
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Class4.php b/tests/php/ORM/DataObjectDuplicationTest/Dingo.php
similarity index 61%
rename from tests/php/ORM/DataObjectDuplicationTest/Class4.php
rename to tests/php/ORM/DataObjectDuplicationTest/Dingo.php
index 8d5aad1e4..879ecbd5d 100644
--- a/tests/php/ORM/DataObjectDuplicationTest/Class4.php
+++ b/tests/php/ORM/DataObjectDuplicationTest/Dingo.php
@@ -11,19 +11,23 @@ use SilverStripe\ORM\ManyManyList;
  * @method ManyManyList Children()
  * @method ManyManyList Parents()
  */
-class Class4 extends DataObject implements TestOnly
+class Dingo extends DataObject implements TestOnly
 {
-    private static $table_name = 'DataObjectDuplicateTest_Class4';
+    private static $table_name = 'DataObjectDuplicateTest_Dingo';
+
+    private static $cascade_duplicates = [
+        'Children',
+    ];
 
     private static $db = [
         'Title' => 'Varchar',
     ];
 
     private static $many_many = [
-        'Children' => Class4::class,
+        'Children' => Dingo::class,
     ];
 
     private static $belongs_many_many = [
-        'Parents' => Class4::class,
+        'Parents' => Dingo::class,
     ];
 }
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Elephant.php b/tests/php/ORM/DataObjectDuplicationTest/Elephant.php
new file mode 100644
index 000000000..75df638f4
--- /dev/null
+++ b/tests/php/ORM/DataObjectDuplicationTest/Elephant.php
@@ -0,0 +1,29 @@
+<?php
+
+
+namespace SilverStripe\ORM\Tests\DataObjectDuplicationTest;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+
+/**
+ * @method Antelope Parent()
+ * @method Frog Child()
+ */
+class Elephant extends DataObject implements TestOnly
+{
+    private static $table_name = 'DataObjectDuplicateTest_Elephant';
+
+    private static $cascade_duplicates = [
+        'Child',
+    ];
+
+    private static $db = [
+        'Title' => 'Varchar',
+    ];
+
+    private static $has_one = [
+        'Parent' => Antelope::class,
+        'Child' => Frog::class,
+    ];
+}
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Frog.php b/tests/php/ORM/DataObjectDuplicationTest/Frog.php
new file mode 100644
index 000000000..95d00856a
--- /dev/null
+++ b/tests/php/ORM/DataObjectDuplicationTest/Frog.php
@@ -0,0 +1,25 @@
+<?php
+
+
+namespace SilverStripe\ORM\Tests\DataObjectDuplicationTest;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+use SilverStripe\ORM\ManyManyList;
+
+/**
+ * @method ManyManyList Children()
+ * @method ManyManyList Parents()
+ */
+class Frog extends DataObject implements TestOnly
+{
+    private static $table_name = 'DataObjectDuplicateTest_Frog';
+
+    private static $db = [
+        'Title' => 'Varchar',
+    ];
+
+    private static $belongs_to = [
+        'Parent' => Elephant::class,
+    ];
+}
diff --git a/tests/php/ORM/DataObjectDuplicationTest/Goat.php b/tests/php/ORM/DataObjectDuplicationTest/Goat.php
new file mode 100644
index 000000000..e18c0c6ea
--- /dev/null
+++ b/tests/php/ORM/DataObjectDuplicationTest/Goat.php
@@ -0,0 +1,23 @@
+<?php
+
+
+namespace SilverStripe\ORM\Tests\DataObjectDuplicationTest;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+
+/**
+ * Note: Not duplicated
+ */
+class Goat extends DataObject implements TestOnly
+{
+    private static $table_name = 'DataObjectDuplicateTest_Goat';
+
+    private static $db = [
+        'Title' => 'Varchar',
+    ];
+
+    private static $belongs_to = [
+        'bobcats' => Bobcat::class,
+    ];
+}