FYI: this is from the indiegala gameplay giveaway thing, it used to be that 9 out of 10 times the key was ok, but recently it has been more like 7 out of 10... it could well be a dud.

Unfortunately there's no way to tell it is a dud until one attempts to add it to an account which does not have the game/dlc

This is not a "PLEASE LET ME DELETE THE GIVEAWAY IN CASE IT DOESN'T WORK!!" thing, it's just a heads up.