Possible, but I'm unconvinced. Every copy of Black Mist one has is one less card they have to actually be more effective in battle and not get into hopeless situations as often; I'd think two copies of it would be worse than one for that reason, as surely it's diminishing returns. If anything, I'd suggest limiting Black Mist to one copy for the exact opposite reason - it's too WEAK to offer to the player once they already have one. If for no other reason than to SHUT UP all the whiners about "discovered mate", Black Mist is practically necessary; to the expert, it's a chance to escape when the game gets truly ridiculous (and sanity knows Black Mist often doesn't get one out of trouble, only delaying it a turn). To each their own, but I don't think it needs a drawback.