Sometimes (approximately once every few months), for one of my email accounts, Thunderbird for some reason decides that all the messages in all the folders need to be re-downloaded. It doesn't give me any indication of this if I don't select a folder, I think the count of unread messages is still displayed properly and gets updated when new messages get moved to the folder, though I don't remember for sure. The way that I notice that the problem is happening is that as soon as I select any folder, all the messages disappear and then start quickly reappearing, and the status bar at the bottom of Thunderbird indicates that it's downloading i of N messages (at some point it stops saying this, maybe after it has downloaded the message headers, but before it has actually downloaded the messages (I have it configured to download all messages from all folders), and the progress bar at the bottom is still going, it just doesn't say what it's doing). I use Thunderbird with a bunch of different email accounts (all via IMAP), and this problem has only happened with outlook.com accounts. I have three outlook.com accounts, one (call it account A) of which I use more than the others, and that's the one that the problem usually happens with - I don't think it had ever happened with either of the other two, until today when it happend for both account A and one of the other accounts (call it account B). When it happened this time, I tried making a backup copy of one of the MBOX files (in account B) from the Thunderbird profile, before selecting the corresponding folder in Thunderbird (causing it to re-download all the messages). Then I compared the old MBOX file to the new one. The new one is actually just two copies of the old one, one after another, identical except that in the second copy, the "From " line at the beginning of each message, which seems to be an indication of the time at which the message was downloaded, has the date/time from today in place of whatever it was in the first copy. So then I went and looked at the MBOX file for one of the folders in account A (the account for which I know this problem has happened multiple times), and I found that that MBOX file appears to be *eleven* copies of approximately the same thing, one after another, and in large part they're identical except for the "From " lines, but there are some other differences as well, I can't tell what the pattern is. So ... any idea what could be causing this or what I should do to figure it out? My Thunderbird version is 52.8.0, on Ubuntu 16.04, and I do have some Thunderbird extensions installed (ImportExportTools, Manually sort folders, SmartTemplate4, and IMAP Received Date).