x402 Settlement Review: Replay and Double Charge — Reviews pasted seller-side x402 code that accepts the PAYMENT-SIGNATURE header, talks to the facilitator and delivers the goods (the cash register), for the silent failures that double-charge a buyer, hand the goods to a stranger or bill the seller. It flags an expiry check that runs before the nonce lookup, a repeat path that delivers without proof the chain observer lacks, a repeat window taken from the buyer's validBefore instead of the sell…

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