I spent an embarrassing amount of time debugging an SQS consumer that was processing messages twice and sometimes three times. The root cause turned out to be a fundamental misunderstanding of how visibility timeout, dead-letter queues, and consumer concurrency interact. Here is everything I wish I had known from the start. SQS Delivery Guarantees: At-Least-Once, Not Exactly-Once Before we dive into configuration, the most important thing to internalise: SQS guarantees at-least-once delivery . The same message may be delivered multiple times, even under normal operating conditions — not just during failures. This is not a bug; it is a deliberate design decision that enables SQS to provide high availability and scalability. Your consumer must be idempotent. Visibility Timeout: The Most Misunderstood Setting When you call ReceiveMessage , SQS makes each returned message invisible to other consumers for the visibility timeout duration. If you do not delete the message before the ti...