Idempotency & Duplicate Prevention When this skill applies Use this skill when: - Implementing any PPP endpoint handler that processes payments, cancellations, captures, or refunds - Ensuring repeated Gateway calls with the same identifiers produce identical results without re-processing - Building a payment state machine to prevent invalid transitions (e.g., capturing a cancelled payment) - Handling the Gateway's 7-day retry window for status payments Do not use this skill for: - PPP endpoint response shapes and HTTP methods — use - Async callback URL notification logic — use - PCI complianc…