check if user key already exists

make sure user attribute keys are imported just once.

this check already exists in this file https://github.com/concrete5/concrete5/blob/master/web/concrete/core/models/attribute/key.php#L232-L233

However, UserAttributeKey overrides the import method and doesn't have a similar check. I've added it here, just in case we want to import something which belongs to a custom attribute key category.