When calling `duplicate` on a form, a form field, or suchlike, the results
would be that all related items to the thing being duplicated (such as
fields for a form, or options to an editabledropdown) would be duplicated
_twice_; ie. where a form had two fields, it's new duplicate would have
four (each one occurring twice). We have stopped this in a backwards
compatible way - that is the bug was introduced with core 4.1, and this
change leaves the userforms module compatible with 4.0.
