Check when edit if the field is not duplicate Entry

if($this->input->post('user_name') != $original_value) {

   $is_unique =  '|is_unique[users.user_name]'

} else {

   $is_unique =  ''

}

$this->form_validation->set_rules('user_name', 'User Name', 'required|trim|xss_clean'.$is_unique);