Under what conditions is multiple message delivery by the broker possible?

Multiple message delivery by the broker is possible when using the "at-least-once" delivery model. This occurs if:

• The recipient does not acknowledge receipt of the message, and the broker resends the message.
• There is a network or recipient-side failure, causing the acknowledgment not to be received.
• The broker is configured to retry delivery to ensure reliability.

In such cases, the same message may be delivered multiple times, so it is important for the consumer to implement idempotent message processing to avoid duplicate effects.