public function duplicate_form_fields($form_id, $ids)
 {
     if (is_array($form_id)) {
         foreach ($form_id as $formId) {
             $fields = RM_DBManager::get_fields_by_form_id($formId);
             foreach ($fields as $field) {
                 $this->duplicate_field($field->field_id, $ids[$formId]);
             }
         }
     } elseif ((int) $form_id) {
         $fields = RM_DBManager::get_fields_by_form_id($form_id);
         foreach ($fields as $field) {
             $this->duplicate_field($field->field_id, $ids[$form_id]);
         }
     } else {
         throw new InvalidArgumentException("Invalid Form ID '{$form_id}'.");
     }
 }