The third most expensive x402 mistake: retrying a payment that already succeeded

The x402 protocol involves several costly mistakes. Treating verify as settlement is the second most expensive error, as verify only checks authorization while settle moves funds. The third mistake is retrying payments that already succeeded, which often leads to double-payments via fresh EIP-3009 authorizations. The fourth mistake is paying without a spending policy, where valid individual payments accumulate into unauthorized total bills because agents lack aggregate caps to stop spending automatically.