Delivery can be repeated
A sender may retry an event if a response is delayed or lost. Receiving the same event again should not automatically repeat the business action behind it.
Separate an event from its effect
An idempotent consumer recognizes an already handled operation and avoids producing an additional effect. A design needs to consider concurrent attempts, not just a check made before processing.
Define the contract
Document how events are identified, what is retried, and how long deduplication records are retained. Ordering and retry behavior must be understood for the provider involved.
Explore the underlying concepts.
For more technical context, read AWS Lambda: Application design and idempotency ↗. This link is a reference, not a statement of partnership or integration. This introduction is not a complete implementation guide or advice about regulated financial services.