What is at-least-once delivery and what does it guarantee?

At-least-once delivery guarantees that a message will be delivered to the recipient at least once. This means that the message will not be lost, but it may be delivered multiple times (duplicates are possible).

This approach is used in messaging systems where data loss is unacceptable, but handling duplicates should be managed at the application level.

Example: if a service sends notifications, at-least-once delivery guarantees that the user will receive the notification, possibly multiple times if delivery confirmation was not received in time.