Some people are experiencing an issue with their Crysis 2 multiplayer key and also see the message "This key is already is use". A patch to fix issue is coming but in the mean time here is an easy work around solution;
1. Go to multiplayer and type in your key
2. Navigate back to Single Player
3. Go back into multiplayer and re-enter your key
After completing these 3 steps your multiplayer key will be saved and you will not be required to re-enter it.