silverstripe-userforms/code/formfields/UserFormsCheckboxSetField.php

<?php

/**
 * @package userforms
 */
class UserFormsCheckboxSetField extends CheckboxSetField {

	/**
	 * jQuery validate requires that the value of the option does not contain
	 * the actual value of the input.
	 *
	 * @return ArrayList
	 */
	public function getOptions() {
		$options = parent::getOptions();

		foreach($options as $option) {
			$option->Name = "{$this->name}[]";
		}

		return $options;
	}

	/**
	 * @inheritdoc
	 *
	 * @return array
	 */
	public function getSourceAsArray()
	{
		$array = parent::getSourceAsArray();

		return array_values($array);
	}

	/**
	 * @inheritdoc
	 *
	 * @param Validator $validator
	 *
	 * @return bool
	 */
	public function validate($validator)
	{
		// get the previous values (could contain comma-delimited list)

		$previous = $value = $this->Value();

		if (is_string($value) && strstr($value, ",")) {
			$value = explode(",", $value);
		}

		// set the value as an array for parent validation

		$this->setValue($value);

		$validated = parent::validate($validator);

		// restore previous value after validation

		$this->setValue($previous);

		return $validated;
	}
}