['email', 'unique', 'targetClass' => User::class, 'filter' => function ($query) {

                if (!$this->getModel()->isNewRecord) {
                    $query->andWhere(['not', ['id' => $this->getModel()->id]]);
                }
            },'message' => 'This email address has already been taken.'],   
            
            
            
            
            
               /**
 * @return User
 */
public function getModel()
{
    if (!$this->model) {
        $this->model = new User();
    }
    return $this->model;
}