x402 middleware verifies, runs the handler, then settles. Verification
does not consume the authorization, so concurrent copies of one signed
payment each run the handler; only one settles. Document a claim-once
guard keyed on the EIP-3009 (from, nonce) or the SVM signed message,
with an atomic insert before the payment middleware, failing open.
Mirrored in ja-JP and zh-CN.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
