code: private function addComboNotUniqueError($model, $attribute)
      {
          $attributeCombo = [];
          $valueCombo = [];
          foreach ($this->targetAttribute as $key => $value) {
              if (is_int($key)) {
                  $attributeCombo[] = $model->getAttributeLabel($value);
                  $valueCombo[] = '"' . $model->$value . '"';
              } else {
                  $attributeCombo[] = $model->getAttributeLabel($key);
                  $valueCombo[] = '"' . $model->$key . '"';
              }
          }
          $this->addError($model, $attribute, $this->message, [
              'attributes' => Inflector::sentence($attributeCombo),
              'values' => implode('-', $valueCombo),
          ]);
      }
docs: Builds and adds [[comboNotUnique]] error message to the specified model attribute.
  @param \yii\base\Model $model the data model.
  @param string $attribute the name of the attribute.