It is a known issue with Thunderbird 3.x that it downloads duplicate emails. I installed Thunderbird 3.1.5 last month (it's awesome!) and configured it to not download duplicate emails using the following well-known solution, this all worked fine until last week:
- set Tools->Options->ConfigEditor->Advanced:mail.server.default.dup_action = 1
- (Email account)->Server Settings: enable 'Leave messages on server', 'Until I delete them'
Also as a manual fallback I installed the add-on 'Remove Duplicate Messages (Alternate) 0.3.5' by eyalroz, which works well.
Everything worked fine until last week (you never see duplicates because Thunderbird deletes them on-the-fly), then last week it started getting duplicates again, despite the settings. We're talking downloading 9000 duplicate emails here every single time it checks mail, it's nearly unuseable. (FYI I access Hotmail via POP. On Vista.) (Yes I already read the Help, FAQ, Release Notes, Bugzilla, superuser.com archives and googled the web. Many reports, few solutions and none that work 100%.)
In the Thunderbird Bugzilla, there are many bugs over many years for 'duplicate email', but the main culprit for my issue might be Bug 608242 - "Hotmail still downloading duplicate messages". Then again it might not be, it might also be 284526, 591334, maybe 612136 or other. To get these bugs more prominence, register on Bugzilla and vote for them. There are many other manifestations of duplicate mail in TB, so do not start arguing that the culprit is Hotmail and not Thunderbird itself. TB should be able to handle this robustly, no matter how crappy Hotmail might be. The underlying root-cause sounds like the hash file 'popstate.dat' in which TB keeps a hash of message date, time, sender, subject etc. and checks it before downloading. Many of the Bugzillas suggest popstate.dat may be being corrupted, e.g. by (AVG) antivirus. There are also reports that filtering or attachments also tickle this. Anyway as I commented, this all used to work fine for me till recently.
Any solutions, workarounds or suggestions?