如何避免Mongo数组中的重复对象

Question

I'm trying to create tasks collection where every user has his own object and his tasks are divided into different days. Unfortunately when I try to add new task for existing day, mongo duplicates the day object. I use facile-it/mongodb-bundle for Symfony

That's my code:

$collection = $this->mongoConnection->selectCollection(Table::READ_MONGO_USER_TASKS);

if($collection->countDocuments(['user_id' => $userId->toString()]) === 0) {
    $this->createUserDocument($collection, $userId);
}

$task = [
    'id' => $event->taskId()->toString(),
    'title' => $event->title()
];

$collection->updateOne(
    [
        'user_id' => $userId->toString()
    ],
    [
        '$addToSet' => [
            'days' => [
                'date' => $deadlineDate,
                'tasks_list' => [$task]
            ]
        ]
    ]
);

And when there are 2 or more tasks for the same day, I receive something like this: compass view

What should I do to make it work as expected?