You have moved your library or deleted it accidentally? You lost the „Already watched“ flag?
Just in case you have a keyboard attached: Go to the movie list and just press „w“ for „watched“. This will take a couple of seconds and then set it to watched.
Absolutely annoying. Even if you’re having the scraper set to „local nfo only“ this will probably hit you. You could set the corresponding flag in the advancedsettings.xml but this will only help when exporting/importing the settings. But it’s probably still going to mess everything up.
So just hit „w“ for all the movies you’ve watched and you’re fine.