12.19Reused key with different payload

First request:

Idempotency-Key: REQ-123

amount = 100

Second:

Idempotency-Key: REQ-123

amount = 900

You should not silently return:

previous $100 result

as though the second request were valid.

Better:

same key
different request hash
→ conflict/error

This protects against client bugs.