Crytek are already hard at work on a patch for the PC version of Crysis 2 that will end the problems some users are having with multiplayer key code.
Apparently some users have reported having problems entering their code that enables multiplayer in the game. Users have reported getting a message stating “This key is already is use.”
Though they haven’t suggested when the patch will be available, Crytek have offered a work around in the meantime.
– Go to multiplayer and type in your key
– Navigate back to Single Player
– Go back into multiplayer and re-enter your key
Thanks to CVG for the info.