Why?!

Huh. Sounds seriously strange. Maybe the Serious Sam HD: The First Encounter somehow evolved or perhaps the gifter got you Serious Sam HD: Double Pack?

Maybe you bought the bundle?:P

So you are the one who hacked steam...
Reported.

Do you share your computer (I'm not talking about the new sharing feature) with someone who owns SS:SE?

Does it show the "Install" or the "Purchase" button in your client?

If in doubt, you can always check your purchase history on Steam to see what has been activated when on your acount.

I was gifted the key for Second Encounter HD and it appeared in my library but not in the licenses. Then I bought the bundle and activated the second key to get the license. And Steamgifts still doesn't see that I own the game. Second Encounter is messed up.

I also have Serious Sam HD: The Second Encounter in my library in my client, but not on my web profile. I never bought it. I installed it and when I try to play it Steam see it as Serious Sam HD: The Second Encounter Demo.