The Idempotency Key (https://lob.com/docs#idempotent-requests) is a feature that allows you to pass a unique key along with each request and guarantee that only one mailer is created and sent. You can safely retry the same request with the same idempotency key and be assured that no duplicates are created even if the API is called multiple times within 24 hours.
For more help integrating idempotency keys, please refer to our documentation and our implementation guide.