# Idempotency & Duplicate Protection

**Document Version:** 1.0.0 (Phase 21)

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## 1. Multi-Layer Idempotency

1. **Email Delivery:** Idempotency keys (`idempotency_key`) recorded in `mail_logs` prevent duplicate email dispatches.
2. **REST API:** `Idempotency-Key` headers cache and replay exact API responses for safe client retries.
3. **Webhook Ingestion:** Provider event identifiers ensure exact-once lead creation.