Fix bug related to user field groups

The previous logic was calling `register_field_group` twice for every `data.php` file you had *if* you project did not have a child theme.  Basically, the problem was that the `$paths` variable had duplicate paths for `active_theme` and `active_child_theme`.  For field groups on custom post types, this actually didn't cause any problems.  However, if you create a field group for your users, it would end up resulting in the user form double rendering all of the custom fields.

The change here makes it so that all `$files` are collected, duplicates removed, and then processed individually.