I think that's the crux of your misunderstanding here. There are networking issues where one side thinks a request has been sent or received successfully while

I think that's the crux of your misunderstanding here. There are networking issues where one side thinks a request has been sent or received successfully while the other side does not. In this specific case, if a networking issue prevents the final ACK from being received by the server then the client assumes the connection is closed and the message has been delivered, while the broker will wait for the connection to time out, which I assume is a failure condition. There are other cases where the client can receive a complete message but the server is unsure that it has, and will time out the connection while the client continues processing, assuming everything is fine. You cannot build something on top of a networking stack that does not guarantee exactly-once semantics at any level (TCP, IP, HTTP, physical) and expect to get exactly once. You can mitigate it, sure, so maybe Amazon is super-dooper sure that these networking conditions won't ever happen.