1. I bought two copies of a game (not on Steam, but activated on Steam). 2. I tried to put them both on my account, assuming the second would be added to my inventory and I could gift it to a friend. 3. Steam not only didn't add it to my inventory; Steam DEACTIVATED the second code (marked it as redeemed), even though I already owned the game.
I gave it to my friend, and was embarrassed when he replied that I sent him an already activated code.
I bought two legitimate copies of the game, Steam. Luckily, the retailer was willing to send me yet a third code that I could send to my friend directly.
But really, don't you think it would have been nicer to warn me that I already own the game and can't add another copy to my giftable inventory, rather than unilaterally deactivating my legitimately purchased second copy?