Serial keys you use in Origin games are single use and when you use one, while being logged on to an Origin account, the game associated with the serial key binds itself to that account. After this the serial is useless for anything other than retrieving your account should it be hacked or lost.
If you’re using an Origin client on any computer after that – log into the account, then you will see the game there without the need for you to enter any serial key. This is how serial keys work in Origin, Steam and practically everywhere else you have multiplayer games and it has been like this for ages.
To sum up:
Serial Keys / Game Keys: can only be used once -> after that the game is on your account.
Hope this helps you in your future endeavours to kill virtual people 
