Deduplicate mails from a set of maildir folders.
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Maildir Deduplicate

Command-line tool written in Python to deduplicate mails from a set of maildir folders.


This script reads all mails in a given list of maildir folders and subfolders, then automatically detects, lists, and optionally deletes any duplicate mails.

Duplicate detection is done by cherry-picking certain headers, in some cases doing some minor tweaking of the values to reduce them to a canonical form, and then computing a digest of those headers concatenated together.

Note that we deliberately limit this to certain headers due to the effects that mailing list software can have on not only the mail header but the body; it can potentially:

  • append a footer to a list body, thus changing the Content-Length header

  • create a new path described by the Received headers which would not be contained in any copy of the mail saved locally at the time it was sent to the list

  • munge the Reply-To header even though it's a bad idea

  • add plenty of other random headers which a copy saved locally at sending-time would not have, such as X-Mailman-Version, Precedence, X-BeenThere, List-*, Sender, Errors-To, and so on.

  • add a prefix to the Subject header

Another difficulty is the lack of guarantee that Message-ID is unique or even present. Yes, certain broken mail servers which must remain nameless are guilty of this :-(

For added protection against accidentally removing mails due to false positives, duplicates are verified by comparing body sizes and also diff'ing the contents. If the sizes or contents differ by more than a threshold, they are not counted as duplicates.

So far, it was tested on:

  • MacOS X 10.6 with Python 2.6.2,
  • Linux with Python 2.6.0 and 2.7.2.



This script was initially released in 2010, and was living in a messy GitHub repository. After some years, the script basically outgrew its initial intent, and moved in 2013 to its own repository.


This code is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2, or any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

For full details, please see the file named COPYING in the top directory of the source tree. You should have received a copy of the GNU General Public License along with this program. If not, see