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Communications RCS

All Outbound Messages Must Be Deduplicated Before Send

7 min read

A message send job that runs multiple times must not deliver duplicate messages to the recipient. Deduplication must be enforced at the send layer, not the job layer alone.

Rule

Message sends must be deduplicated by a stable send key before delivery.

Why

Queue retries, job replays, and worker crashes can cause the same message to be sent multiple times.

Must

  • Compute a deterministic send key from (recipient, template, campaign, date-bucket).
  • Check the send key against a sent-messages record before dispatching.
  • Record the send key atomically with the dispatch call.
  • Expire send keys after a deduplication window (24-72 hours).

Should

  • Use a unique message ID provided to the carrier so they can also deduplicate on their end.
  • Log duplicate sends as a metric and alert on spikes.

Anti-patterns

  • Deduplicating only at the job level with no send-layer check.
  • Send keys that include a timestamp component shorter than the dedup window.

Test Cases

  • Replaying the same send job twice delivers one message.
  • Expired send key allows re-send in the next dedup window.

Telemetry

  • message_send_deduplicated
  • message_send_dispatched
  • deduplication_spike_alert