Duplicate messages received

Some people have experienced problems with Thunderbird receiving duplicate messages when downloading mail. In other words, messages that already exist in one's Inbox (or elsewhere locally) are downloaded again. Typically the duplicates appear in one's Inbox as unread messages. Depending on the user, it may happen always when downloading, only sporadically, or perhaps one time only. It may happen with some mail servers but not others, after a previous mail download that was interrupted (for instance, if Thunderbird crashed while downloading), or after reinstalling Thunderbird.

Deleting duplicates: If you have received a huge amount of duplicates all at once, one way to remove them from your Inbox is to sort messages by the order they were received (View | Sort by | Order Received). Since the duplicate messages were all received at once, this should group them together in the message list. You can then select all of them together and send them to the Trash folder.

To avoid receiving further duplicates: There is no sure-fire method to ensure that you will never receive duplicate messages again, but below are some things you can try, with the easiest options listed first.

• Set Thunderbird to delete messages from the server: go to Tools | Account Settings | Server settings, and uncheck "Leave messages on server."

• Compact your mail folders on a regular basis: for each mail account you have, select the Inbox and go File | Compact Folders.

• Compact folders, exit Thunderbird, and go to your mail folders. Delete the index files, named *.msf (such as Inbox.msf). These will be rebuilt when your launch the application again. (You should not need to do this on a regular basis, however.)

• Try deleting the popstate.dat file from the account subfolder in the Mail folder in your profile folder. If popstate.dat gets corrupted Thunderbird can get confused about what has been downloaded. (Note: if you delete popstate.dat and you still have messages on the server that have already been downloaded once, they may all get downloaded again.)

• Create a new profile and migrate your mail to the new location. (See the "File Locations and Migration" section on this page for more information.)

Of course, if you try deleting files or making a new profile, it's wise to first make a temporary backup of your mail and settings.