Nobody cares. These are short lived transactions (generally milliseconds); collisions are a rare edge case; it's fine to block. One request succeeds, the other gets a dup error (or a replay).
You could invent your own more sophisticated idempotency API but good luck finding someone that wants to implement it or use it. What real-world problem are you trying to solve?
You could invent your own more sophisticated idempotency API but good luck finding someone that wants to implement it or use it. What real-world problem are you trying to solve?