Perhaps, perhaps not. At the very least one could admit to a certain level of mismanagement.
edit: Also, people making that mistake would explain why some folks are downloading multiple gigs of files. I have some doubts that they will get their cash back from steam. No doubt some would argue, they get the 'unique items' associated with a steam purchase. *shrugs*
That doesn't make much sense, because every legal copy of the game has to register with Steam, and Steam gives them adequate warning. Not only would it say "You already own this game", but the page would be on New Vegas, the price tag would clearly say 50 dollars, etc.
I don't have any sympathy for anybody who somehow buys a second copy of the game that way. I mean really, it is very obvious that you aren't buying Dead Money in this situation.