See, that's something that doesn't really set well with me. Typically even games very lax on DRM won't allow two people with identical keys to play together.
I have a question about the serial key actually. Would the game actually allow two players with identical serials play a game together? If not, would there actually be a problem if that were actually not allowed to happen?
1) Yes, two people with identical keys can currently play together. I know, as I did this with the friend who introduced me, prior to buying my own copy.
2) Actually, yes. If you activate with one key, there's no way to "revoke" that key and/or enter a new, different one (ditto expansions).
I didn't say it was a good/optimal solution, just that that is the way AI War is.
(Oh, and btw, Age of Empires works this way too, back when games came on CDs. You needed the CD to start the game, but after that you could take it out and transfer it to another computer, and the two instances could mutliplayer together).