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: