I see 2 options there:
Create additional internal field in the model, something like
private $task_sended = false;
In afterUpdate set task_sended = true and check Itif (!$this->task_sended){ //send task to scheduler $this->task_sended = true; }
Use DAO command to update model
\Yii::$app->db->createCommand()->update(self::tableName(), $update, ['id'=>$this->id])->execute();