Reasoned resolution, without executed tests: claim the scoped token under a database uniqueness constraint before applying business writes, and commit the claim, writes, and replayable outcome in one transaction. A competing attempt must resolve that transaction before proceeding: after commit it replays the outcome; after rollback it can claim the token and execute. A process crash after durable commit but before acknowledgment therefore leaves enough information to return the original outcome without repeating effects. Splitting the effect and receipt across commits breaks this invariant. Retention bounds the guarantee unless expired requests remain rejectable. For effects outside the transaction, an outbox provides durable delivery intent but duplicate delivery still requires receiver-side deduplication or equivalent atomic cooperation.

The reasoning now includes canonicalization changes during schema rollout. Store an immutable fingerprint-version identifier and replay contract with each transactional receipt; keep operation-key uniqueness independent of those versions. For an existing receipt, its version selects comparison rules. Parse the retry under its declared request contract, including historical defaults, and permit conversion only when every effect-relevant distinction is preserved or explicitly checked for equivalence. Blindly dropping fields unknown to an older canonicalizer can accept a changed operation. Conversely, a checked field rename or explicit representation of a historical default can safely replay across versions. Concurrent claim losers compare against the winning receipt after commit regardless of their preferred write version. Deploy compatible readers before emitting new receipt versions, and reject or route unsupported versions without re-execution. Historical hashes cannot recover distinctions an old canonicalizer already discarded; retained request evidence or rejection is required for ambiguous comparisons. These are deductions from the transactional invariant, not executed tests.