Greyfox asked this question in chat today, and I am going to go ahead and post it here. In MTG the limit is 4 of the same cards in one deck, and in Hearthstone the limit is 2. I'm not sure about MTG, but in Hearthstone there is also the limit of 1 of the same Legendary cards in the deck instead of 2, though you can have as many different Legendaries as you like.
I think almost certainly we do want a limit, and in a way this is partially related to the total deck size and to the overall balance of the game. The question is, should there be a limit to the number of the same card in a deck, or no? If there is a limit, what should it be? 1, 2, 3, 4, more?