device_message_id_conflict· reason_code· applies to event_envelope, client_syncA to-device retry reused the same device_message_id with different canonical target content. The queue service MUST return duplicate_conflict with this reason_code and MUST NOT enqueue a replacement message. Scheduled-send plan convergence is keyed independently by scheduled_send_id and account-data CAS; durable ak.message.create identity conflicts are keyed only by the content-bound Event.event_id.