if i want to activate Dark Messiah for multiplayer i somehow can register it says:

Duplicate Product Code

The product code you've entered has already been activated by an existing Steam account, and is therefore invalid. Your activation of Dark Messiah has not been completed.

can someone help me plz got it just a few days

I've got the same problem mate.
The bastards from the store stole the CD key from the game.

You need to take a picture of the game's CD key,the store ticket(so that they can be sure you bought the game) and send to steam.But go to steam site,they explain everything there.

i tried everything what they explained still nothing :S