If it is just some old messages, a possible workaround is to move the affected messages into another folder and then back again. This will cause them to get a new UID on the server.
If that doesn't fix it in thunderbird, you may need to close thunderbird and do the moves on the web interface. When you re-open thunderbird it won't know that it's the same message, so will re-download it. That should definitely fix it
Bron.