code: public function init()
      {
          parent::init();
          if ($this->message !== null) {
              return;
          }
          if (is_array($this->targetAttribute) && count($this->targetAttribute) > 1) {
              // fallback for deprecated `comboNotUnique` property - use it as message if is set
              if ($this->comboNotUnique === null) {
                  $this->message = Yii::t('yii', 'The combination {values} of {attributes} has already been taken.');
              } else {
                  $this->message = $this->comboNotUnique;
              }
          } else {
              $this->message = Yii::t('yii', '{attribute} "{value}" has already been taken.');
          }
      }
docs: {@inheritdoc}