Steam now has trading cards. In order to get them, you're supposed to play specific games (currently there are only 6 games supported, but they've said more will come).
However, it's only possible to find half of the total trading cards for each game - you have to trade for the other half.
Can I find the same trading card twice in the same game (and if so, will that count against my allowed card-pickups)?
Is it possible to trade for a duplicate of a card you already own?