protected function getUniqueListProductClassField()
    {
        $types = $this->field->getTypes();
        $fields = $this->getListProductClassField(array('index' => 'field_id'));

        $unique = array();

        foreach ((array) $this->field->getList() as $field) {
            if (empty($fields[$field['field_id']])) {
                $type = empty($types[$field['type']]) ? $this->text('Unknown') : $types[$field['type']];
                $unique[$field['field_id']] = "{$field['title']} ($type)";
            }
        }

        return $unique;
    }