The main application for this test is handling duplicate deliveries commonly caused by mailing list subscriptions or redirected mail addresses.
The detection is normally performed by matching the message ID to an internal list of message IDs from previously delivered messages.
For more complex applications, the "duplicate" test can also use the content of a specific header field or other parts of the message.
Duplicate deliveries are a common side effect of being subscribed to a mailing list. For example, if a member of the list decides to reply to both the user and the mailing list itself, the user will often get one copy of the message directly and another through the mailing list.