x402_payment_replay

HTTP 409

The same X-PAYMENT authorization was presented twice. Its nonce is a one-time lock, so the second attempt settles nothing. Do not sign a replacement - check whether the first succeeded before paying again.

Response shape

{
  "type": "https://cogdepot.com/problems/x402_payment_replay",
  "title": "<short human-readable summary>",
  "status": 409,
  "detail": "<what happened on this occurrence>",
  "reason": "x402_payment_replay"
}

Branch on reason. The title is prose and may be reworded; reason is the stable contract.