Clone Army, Uncommon, 0-cost
Attack = #instances of selected card in game multiplied by
it's rarity (C=3 U=7 R=17)
I'd like this more if you could target from your opponent's hand, making it as strong a countermeasure to collecting several of one card as it is an incentive. Hydra comes to mind. Hydra doesn't need the boost on the part of the one playing it, but it would be great if this could effectively be used against it.
12-24 damage in response to Flare attack is pretty nice. (Your opponent is likely to already have burning in hand, so you can't expect him to end up with four Searing fire.) I suppose you could use it with the Pallisades you're being given by a Horde deck, but you probably won't get very high values.
I can't think of any other cards you're likely to have several of in a game right now. What are others' thoughts?