The original 'Remove Duplicate Messages' is missing important features, and does not have a free software license. This is a full rewrite, with an GPL-LGPL-MPL license. Performance has been improved and many features and capabilities have been added, including:
- Choice of comparison criteria (subject, author, date, etc).
- Comparison of message bodies.
- Action on multiple folders.
- Review dialog for double-checking which duplicates to delete and which to keep.
- Good performance on local folders with large number of messages; reasonable performance with IMAP and RSS accounts.
- Spiffy toolbar button+menu.
There's also a SeaMonkey version, though not as frequently tested.
I had tried contacting the author of the original 'Remove Duplicate Messages' several times, without success. I would have liked to join forces and have just one extension, but that doesn't seem to work out.
Also, some locales are incomplete - help with them would be appreciated. Visit the BabelZilla translation website.