sure, if you assume that everything can fail, then it doesnt help to store the list of messages you've seen but if you can persist a monotonic sequence number,

sure, if you assume that everything can fail, then it doesnt help to store the list of messages you've seen but if you can persist a monotonic sequence number, thats gets you pretty far. we use tcp all the time even though its has no magic answer to distributed consensus (and uses a super-weak checksum). 2pc doesnt guarantee progress and/or consistency either and its pretty effective.