I am also getting the "already installed" error. I completely removed any prior versions, ran CC cleaner, restarted and tried installing from both the launcher exe and setup exe files, both of which still give the same errors. I searched for hidden files, found one and removed but still no dice.
I can run the old version from the master file however that will not update and will not allow me to update my mods either. Any other suggestions or fixes? Appreciate any help. Thanks!