TCP can't do exactly once in every scenario without fail. It does a pretty good job, but failure in the edge cases is the whole point of the difficulty we're l

TCP can't do exactly once in every scenario without fail. It does a pretty good job, but failure in the edge cases is the whole point of the difficulty we're looking at. If the network loses the last packet in a TCP stream, then goes down for an indefinite period, the two ends of the connection have irreconcilable differences of understanding about whether the entire transmission was received. As far as the recipient is concerned, they have an entire message that's fully ack'd and so they should process it. As far as the sender is concerned, they have no ack for the last chunk of data and must redeliver it. This is the crux of the Byzantine Generals problem. TCP can solve problems that happen in its own domain, and give reliable in-order delivery once over an unreliable network up to a point. It's not able to provide exactly once semantics in all scenarios though. Because that's logically impossible.