Celery - collision of task_ids

I'm getting HardTimeLimit exception for my tasks. After log examination i found -

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1. task is not being received by celery ( No "Got task from broker:" message for task id)
2. task with the same id was executed couple a days ago.

Task ids are assigned automatically by @task decorator, tasks are started by django, there are ~2k tasks per day ( and ~30 collisions per day).

How ID's collision is possible? How to prevent it.

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It shouldn't be possible and even if, it should be very rare. My guess would be that the same task is executed a second time after your exception. Maybe there is a problem with your routing keys as the worker doesn't get the task? Or the broker has a problem, I've seen funny problems with RabbitMQ. Deleting it's database (RABBITMQ_MNESIA_BASE) helped in my case.