Yes, as Oregonflyer mentioned, it's a "bug" in the app. For some reason United added it as an entry field during mobile app check-in but it doesn't bother to check first if you already have a KTN# associated with the reservation. Your KTN# is still there in the background and you'll still get PreCheck privileges WITHOUT re-adding it during check-in. It's another IT fail by United.

Note: this only happens on international reservations for me. It doesn't do it on domestic ones (although I haven't flown domestic for over a month as I'm overseas so maybe they 'enhanced' the app to force it on domestic res too).

-RM