Steam CD keys

Well then my second guess was right.

thank you!

If you own say "Fear 3" that you bought via steam, then tried to use a Fear 3 cd key that you bought at say Amazon, the key would still be good. However, if you did NOT already have a copy of fear 3, then the Fear 3 from amazon would be added to your account.

Once a key is used, its used and not be claimed again. So using Fear 3, if you bought a copy from Amazon, plugged in your code, thus adding it to your account, then gave the code to a buddy, they would get an error. If they already owned the game then they would just get an install prompt.