I want both. If there are two network requests for the same action, I want to be able to trace them distinctly and also know that they are doing one thing.
Also worth noting that a header named 'Request-Id' is already a widespread convention. You'll see one back from many popular APIs like AWS or Stripe, and definitely want a name for the idempotency header that differentiates itself from that.
Maybe "Logical-Request-Identifier" and "Physical-Request-Identifier"?