ilikejuice

My friend and I tried to play Arma 2 multiplayer for the first time, and we were able to connect to empty servers individually; but when the other tried to join the server, it would kick us and the person in the game would get the notification "CD key in use". I also tried hosting a game but it gave the same notice. We both have legitimate copies obtained through steam. Any help with figuring this out would be awesome.

No, we both have legitimate copies of the game. Each bought separately, and both through steam.