Why do I occasionally (several times per week) receive duplicates of previously received e-mail messages?
There are several details to add to the question. Appears random; no connection between senders; everything appears as originally received; no repetition of duplicates; it's always 1, 2, or 3 messages; all have been deleted and "trash emptied"; age varies.
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Are you actually downloading/receiving duplicates OR are previously deleted emails reappearing?
Reappearing emails can mean a corrupt folder and this is not a good idea. so, just in case, do the following to rule it out.
Move all good emails from Inbox into another suitable folder. Delete any you do not want.
right click on 'deleted' and select 'Empty deleted' right click on 'Junk' and select 'Empty Junk'
When Inbox is empty:
right click on 'Inbox' and select 'compact'
right click on 'Junk' and select 'compact'
why compacting is important to maintain a healthy Thunderbird. http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders
Since the messages appear to be duplicates I'm not able to identify their source. However, "reappearing" is the most likely scenario as compared to being downloaded. Thunderbird's context menu (select Properties) provides a repair option for each folder's index file. This program is used periodically and should solve some duplicate issues. Additionally, the context menu provides a "compact" option which should take care of related issues. Do these "repair" programs work? I suppose they do, but there are a myriad of problems that can cause the duplicates problem to resurface.
Right click on folder select 'Properties' provides a repair option for that folder's index file. If the index file gets out of synch with what is actually in the mbox file, then repairing the folder is useful.
When you select to delete an email, it is 'marked as deleted' and hidden from view. You might see it in the 'deleted'/'Trash' folder, but in reality, it is still in the original folder, hidden and using space. This makes it easier to retrieve emails accidentally deleted in error. Compacting means removing all the 'marked as deleted' emails.
Normally, you do not need to actually move good emails and empty the Inbox before compacting. But...If you ever think there might be some corruption on a folder - like deleted emails reappearing, do not compact that folder until you have moved all the good emails. As the risk is a loss of emails on a corrupted folder.
A lack of regular maintenance in compacting can lead to corrupted folders.
In my case, I will delete several emails every day, so my Inbox and my Junk will get compacted daily as they receive the most activity.