It's possible to guarantee exactly-once delivery, just not in bounded time, but eventually. That's what FLP impossibility is about, impossibility to achieve consensus (e.g. exactly-once) in bounded time. But if you are talking in the context of semantics, it's a totally different thing, it's a computation model and it's absolutely possible to have exactly-once semantics, regardless whether you achieve it by waiting, like consensus-based systems, or by using special data types and operations that can be performed without waiting and eventually converge, like CRDT systems.