I had something that sounds similar - a number of emails that had previously been downloaded from a gmail pop3 account suddenly reappeared in my Thunderbird inbox. These emails had previously been moved by message filters to Local Folders within Thunderbird. I am 100% convinced that the emails were NOT being redownloaded but rather they were never properly removed in the first place! Thunderbird has a folder index (.msf) file separate to the stored emails. There seem to be circumstances when emails are removed from the folder index (and therefore do not show up in Thunderbird) but the emails themselves are still stored on disk. In this case the folder is classed as 'damaged' and if you manually repair it the emails will reappear (Thunderbird rebuilds the index based on the emails on disk). What I was finding once or twice was that Thunderbird seemed to do this repair automatically (I think following install of a Thunderbird update). The clue that emails are not being removed properly is that the inbox size displayed in Thunderbird (which reflects the stored emails) is not reduced by compacting the folder. I have recently submitted a question regarding moved emails not being deleted but no replies yet.