Serial keys are pretty much what you pay for when you purchase a game. Since if it has singleplayer and multiplayer, the CD key is what is detected that allows you to play multiplayer. If someone else has the same CD key, and you are both trying to connect to multiplayer, a number of things can happen depending on the game. One either gets disconnected, or the second user just cannot connect period, some games will even lock you out of that CD key after repeated attempts.
If you like online play, I wouldn't give out the code.