nodupmail is a procmail-based filtering system, designed to prevent or limit users from receiving duplicate mail messages.
nodupmail is useful for those instances when you get get
duplicate mails because a person mails both you and a list
you are on. This is an often occurence in mailing lists
where the Reply-To:
field is not changed to be list, but
users generally reply back to the list anyways, including
the person they are replying directly too.
nodupmail is more powerful than using formail's built-in duplicate detection because formail relies on detecting duplicates from the Message-ID field of mail messages. This has several conseqeuences: