Use this addon to remove duplicates:

• https://addons.mozilla.org/en-us/thunderbird/addon/remove-duplicate-messages/

If you are repeatedly getting duplicates, there may be an issue with your popstate.dat file. see info:

• http://kb.mozillazine.org/Popstate.dat

You would need to delete the 'popstate.dat file and then Thunderbird will create a new one. The downside is that it will download everything that is on your server again, but just once.

• Close Thunderbird.
• Locate your Profile folders:
• http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Then look in the \Profiles\xxxxxxxx.default\Mail\account name folder for 'popstate', a DAT file.

• Delete the 'popstate.dat' file.
• Open Thunderbird.
• Click on GetMail.

Download will happen and you will need to sort through the mail. This should fix the popstate file so that it has an uncorrupted record of what has been downloaded.

Use the addon to get rid of duplicates.

If you have this setting: View > Folders > Unified

then you will see a Unified Inbox containing all emails from all Inboxes. below there will be the individual mail account Inboxes containing only that mail account Inbox.

If you are saying that you have this 'Unified' view and emails show up in both the Unified folder and the Inbox for the mail account, then they are not duplicates.

Try changing the view to View> folders > All and then you will see if there really are duplicates.