LINE error 409: The retry key is already accepted

You sent a push, multicast, narrowcast, or broadcast request with an X-Line-Retry-Key header, and LINE had already accepted a request carrying that same key. The status table describes 409 as: "An API request with the same retry key has already been accepted." Crucially, this is the idempotency system working, not failing - the 409 is LINE telling you the original send went through, so the duplicate was suppressed and no user got the message twice.

The mechanics: you generate the retry key yourself - "the retry key isn't generated by LINE. Each developer must generate their own retry key" - as a UUID in hexadecimal notation, and attach it to the first attempt. If that attempt times out or errors ambiguously, you retry with the same key; a genuine failure is re-processed, while an already-accepted request returns this 409 along with the x-line-accepted-request-id header carrying the request ID of the accepted original, and - for push - the sentMessages array of the original send. So a 409 handler should treat the operation as succeeded and record the accepted request ID. The retry key exists precisely for ambiguous failures like timeouts and 500 Internal Server Error.

The window is finite: "the retry key management period on the LINE Platform side is 24 hours. If you use the same retry key for more than 24 hours, the request will be treated as a new API request" - and LINE warns explicitly that reusing a key past 24 hours "will succeed as a new API request... As a result, duplicate messages will be sent." Retry keys are for retrying one logical send, not for permanent deduplication.

HTTP/1.1 409 Conflict
x-line-request-id: 123e4567-e89b-12d3-a456-426655440002
x-line-accepted-request-id: 123e4567-e89b-12d3-a456-426655440001

{
  "message": "The retry key is already accepted",
  "sentMessages": [
    {
      "id": "461230966842064897",
      "quoteToken": "IStG5h1Tz7b..."
    }
  ]
}

Adopt the pattern LINE designed: persist a fresh UUID with every outbound push/multicast/narrowcast/broadcast job before the first network attempt, send it as X-Line-Retry-Key on every attempt, and map 409 to "already delivered". Keep retry windows far inside the 24-hour key lifetime, and audit that no two jobs can share a key. See retry key limits for the documented window and covered endpoints.