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If it’s a retry of the same request it should have the same key. If it’s not a retry, a different one. I don’t see the issue.

If the client sends the same key but a different payload that’s a 400 or 409 in my eyes.

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It shouldn't, an error would be the right response.
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1) Fresh UUID

2) Client's choice

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now you are moving the core question to where the fresh UUID is calculated. a UUID is calculated or reused based on a process-defined decision.
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