After you save the file remove_duplicate_messages_alternate-0.3.12-tb.xpi on your computer proceed in to your Thunderbird client.
In the system windows select your downloaded file and start the installation by clicking Install now.
After restarting Thunderbird the installation will be completed and you will see a new item Duplicate Messages Deletion in context menu, after right clicking a folder.
Crate a new folder inside your mailbox e.g. "Received duplicities". You can skip this step if you will be just deleting all duplicit messages.
Right click a folder (Invbox) and from the context menu select Duplicate Messages Deletion.
When the analyse is completed a new window with results will open, here you can select how should the duplicities be handled.
- Duplicit messages can be either deleted permanently, moved into the bin or moved to any desired Thunderbird folder.
Finish the deleting (moving/transfer) by clicking OK.
For going through additional folders (IMAP accounts), run the add-on in a same way for the desired folder.