I haven't heard about a lot of double typing issues on the Passport but when it happened on the Q10 a while back it was solved with a new keyboard. I believe that problem was down to a manufacturing error. Not sure if this is the same thing.
My phone is one years old. I bought it as a new phone and it is very strange that just in one year it faced with this problem.
Generally "c" letter has this issue
i guess you may have to get it repaired. when you get "new" one, do not push too hard to get the click (some of them love to get the feedback click feel), and push straight down, instead of sliding (letter C is in the bottom middle, it may have the swipe push action)... if the dome got pushed to the side ways, it may got double click... best of luck. have a wonderful holiday. IMHO.