Quite. The problem with a visibility timeout is that there can be delays in handling a message, and if the original winner fails to do it in time then some oth

Quite. The problem with a visibility timeout is that there can be delays in handling a message, and if the original winner fails to do it in time then some other process/thread will, yes, but if processing has external side-effects that can't be undone then the at-most-once guarantee will definitely be violated. And if you run out of storage space and messages have to get dropped then you'll definitely violate the at-least-once guarantee.