I bought my second copy of WoW from direct2drive (I live in the US) but was not able to buy more. If you try to purchase a title you already bought from them, they tell you it's only possible if you add it to the cart as a gift ... and I couldn't figure out how to do it. So you probably didn't miss much with direct2drive.

I eventually stopped trying, and went to Best Buy to acquire three Battle Chests to go with my old copy and more recent direct2drive copy. Done!