Duplicate Message Remover 0.1

Here's the extension for Thunderbird I was working on. It removes duplicate messages in any given folder and moves them into the Trash (the operation can be undone). The function can be accessed by right clicking on a folder and selecting "Delete Duplicate Messages".

Installation instructions

• Download the extension to your computer (Right-click on the link, and choose "Save Link As" if you're using Firefox. Otherwise, Firefox may try to install it itself)
  
• In Thunderbird, go to Tools->Extensions and click on the Install button. Browse to the file you just downloaded and click on Open.
  
• Restart Thunderbird... and you're done!

By the way, the extension requires Thunderbird 1.0

Known Limitations

• The extension works on folders only. Executing the function on an account will do nothing.
  
• It does not recurse into folders. If a folder contains subfolders you'll need to run it on each subfolder manually.
  
• Since the extension analyzes each folder independently, messages duplicated across folders won't be detected.
  

Thanks to the anonymous poster for the suggestion regarding the message IDs. It turned out that the IDs can be obtained via Thunderbird APIs (they are stored in the metadata database). There's no need to parse the emails at all.

Update: a new version compatible with Thunderbird 1.5 can be found here.