A payments company is building a new serverless webhook receiver that accepts event notifications from a third-party provider. The provider guarantees at-least-once delivery, meaning the same webhook may be sent multiple times. Each event triggers a Lambda function that debits a customer's account balance stored in DynamoDB. During a load test, duplicate webhook deliveries caused some accounts to be debited twice. The architecture must ensure that each unique event is processed exactly once, while remaining highly scalable and requiring minimal operational overhead. Which approach best meets these requirements?