On case insensitive systems it doesn't validate the case of the add-on ID so I can run
xf xf:addon-install ThemeHouse/UIXto install it once
xf xf:addon-install ThemeHouse/Uixto install it a second time
xf xf:addon-install ThemeHouse/UIxto install it a third time
Only time it doesn't let me install it more than once is if I use the exact same case of one of the versions already installed, so if I run
xf xf:addon-install ThemeHouse/UIXagain it'll give me an error
This add-on is already installed. Please upgrade it instead.