This CD key or Activation code is already active on this account.
I received FC4 as a gift for christmas. After the 1 hour install I was prompted for the CD-key which I entered and received the "this key is invalid" message. After entering it exactly the same way 20 more times and getting locked out of UcantPlay a few times in the process it finally accepted the key.
The game did not launch or appear in my games library. When I try to launch FC4 from the system folder it will prompt me for the cd-key again and when entered it will display the message "This CD key or activation code is already active on this account, Please visit your games library to access your games". After launching once from the system folder FC4 will appear in the games library but trying to launch from the library will give exactly the same error.
Same activation problem "Launch from library... blah... blah... blah"
What to do? Buy a different game? No. I like Far Cry, please let me play it. EB Games said FC4 is a great game, alas, I can only take their word for it.