Note that you can in fact solve the exactly-once delivery problem using the model of FLP. It is much easier than consensus, because once message receipt is ackn

Note that you can in fact solve the exactly-once delivery problem using the model of FLP. It is much easier than consensus, because once message receipt is acknowledged it does not has to become a common knowledge. The only serious objection to the idea of exactly-once delivery is that in some models of computation, you can't even solve it on a single node. It may be impossible to process a message and record the fact that this processing have been done in a single atomic step - making any node failure in-between those steps unrecoverable without reprocessing. This is exactly the objection that was raised in referenced article "You Cannot Have Exactly-Once Delivery". Though I don't think it is particularly illuminating observation, especially that it doesn't have anything to do with distributed nature of the system.