Remove Duplicate Messages is a freeware extension for Mozilla’s Thunderbird email client that allows you to search for and remove duplicate email or RSS messages which may be a result of time outs, merging accounts, RSS feed re-download, etc. The extension allows various methods to compare and delete duplicate messages including:

-Message ID
-Date in seconds (precise time in seconds)
-Subject
-Author (sender)
-Line Count
-Message Body

You can choose to search subfolders, trashcan folder and the sent items folder.

To install the addon, download and save the *.xpi extension on your local drive from here. Open Thunderbird, goto Tools –> Extensions –> Select Install and choose the extension from your hard drive