Steam – Why does Modern Warfare 3 download twice on Steam

As several people have mentioned, it is now possible to be logged in on multiple computers at once. Only one computer can be playing a game at a time though. If you start a game while another computer is already in one, you will be notified and told that it will close the other game if you start.

Move the subfolder /steamapps/ in a safe place before uninstalling Steam, then do the following steps:

1. Uninstall Steam
2. Reinstall Steam
3. Launch Steam
4. Exit Steam
5. Move the content of your /steamapps/ backup to the new /steamapps/ subfolder
6. Relaunch Steam

At this point Steam should find all games without any other action; in the worst case, where Steam does not detect some games as installed, just reinstall them and the game will be downloaded and ready in few seconds/minutes because it only need to validate the game content cache.