I'm adding a custom drop down to the userform module and everything seemed to work, however:

If I set the following:
Textfield One - Dropdown option one
Textfield Two - Dropdown option two

It seems to only save the the setting under Textfield One, and ignores Textfield Two. If I then go back and set Textfield Two again, it wipes out Textfield One.

Am I supposed to iterate over something save each value independently or ...?

//config.yml
EditableFormField:
  extensions:
    - CustomEditableFormField

//CustomEditableFormField.php 
class CustomEditableFormField extends DataExtension
{
    public function updateFieldConfiguration(FieldList $fields)
    {
        $fields->push(DropdownField::create($this->owner->getSettingName('CustomTextId'), 'Custom field')
            ->setSource(array("1" => "One", "2" => "Two"))
            ->setEmptyString($this->owner->getSetting('CustomTextId')));
    }