IDs of task can duplicated when mvсс is on

[R-omk]

https://github.com/tarantool/queue/blob/ea8449b6fa06c373124cee0530e71c7fd7a11c78/queue/abstract/driver/fifo.lua#L70-L73

To minimize the possibility of conflicts, MVCC uses what is called best-effort visibility: for write transactions it chooses read-committed, for read transactions it chooses read-confirmed.

by default task_id:max read confirmed , this is the key reason why duplicates occur in the case of an race condition

[better0fdead]

@R-omk Hi, do you have a reproducer?

[R-omk]

I just reported an obvious problem that someone encountered in the telegram chat.

I think you should use require('txn_proxy') to reproduce issue without external client. (or net box client)