The ‘duplicate’ extension (RFC 7352) adds a new test command called duplicate to the Sieve language. This test adds the ability to detect duplications. It is supported by dovecot’s Pigeonhole project. It’s now supported also by Roundcube’s managesieve plugin.
The main application for this new 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.
In tests section new position “Message” has been added. On a message level we can now check if the message “is duplicate” or “is not duplicate”. Additional fields described in the RFC are available in rollover part of the form.