That's still "at most one" – ie. even though state of database itself is "exactly one" (...can "hold the lock") – the fact that actor thread is remote, it means

That's still "at most one" – ie. even though state of database itself is "exactly one" (...can "hold the lock") – the fact that actor thread is remote, it means it can be partitioned/starve the rest of the system. "Exactly once" can be achieved with "at least once" + _local_, reliable state where you can resolve/ignore duplicates. But if that "local" state is actually "remote" – well, then you loose this guarantee (because you can be arbitrarily partitioned from that state so you're in square one again).