Relay A claims outbox row 42 and publishes its message. Before A records completion, its lease expires. Relay B claims row 42 and publishes it again. Both relays behaved according to their local state; the broker received a duplicate. A lease prevents unlimited concurrent ownership. It cannot create exactly-once delivery across a database and broker without a shared transaction. The honest contract is: Every committed outbox event is eventually offered at least once; consumers preserv...