Having dates with the 1970 value. The first of January specifically is a clear sign of file/data corruption. It is the number Zero which should never be the date of an email. But it is a good indicator of garbage in garbage out computing. Now the next issue is how do these duplicate appear. Despite what you say, the fact they have previously been downloaded means nothing, unless they were deleted at the time of downloading, something that can only occur in POP mail accounts and only if the server does what it is told. The Hotmail/Live/outlook one does not do as it is told. Fundamentally most folk that have these issues have one of two constants. A failing hard disk on which the file is stored, or an antivirus product that is corrupting the reads and writes to the disk. How have you recovered from these events in the past? I ask because compacting will clear the corruption, but it is also capable of loosing a vast amount of wanted data. I would suggest accessing the profile folder via the troubleshooting menu and deleting all MSF files in the profile. That is the least risky thing at this point I think, but you will need to have Thunderbird shut down and I strongly suggest you have an exclusion in your antivirus program for the Thunderbird profile folders as it is probably the root cause of your issues over many years. If you are using anything but windows defender as antivirus, try without it and see if that helps. All I have used for several years now is defender without an infection, without the annual expense and without the issues having every file opened right when I want it for Thunderbird to use so it can be scanned.