I think Trasgu hit it on the nail, it's happened to me several times and the same thing what he said works for me as well. It's normally when I'm playing like Trasgu and suddenly the connection fails or whatever, and your kicked. The game still "thinks" you are online and playing so when you try to connect again it tells you your KC is in use. There are times I have waited 5 mins and others longer than 30 mins, I guess it's different but just be patient and it'll eventually work again. Oh, and that's assuming someone doesn't have your key code too, lol.