A LINE push made exactly one attempt, so a transient provider or transport
failure dropped the reply even though retrying was safe to do. Retrying alone
would have duplicated a send LINE already accepted, so every push now carries an
X-Line-Retry-Key and reuses it across attempts: LINE answers a replayed key with
409 and the accepted request's sent messages, which resolves to the original
delivery instead of a second message.
Retries follow LINE's documented policy - server errors and transport failures
only, never 2xx, 409 or any 4xx - and run through the shared channel API retry
runner in strict mode. Replies stay single-attempt because LINE offers no retry
key for them.
Eden