Okay, I've been working on this card for a while, so here it is:
Caltrops - Action Attack Duration, $5
At the beginning of your next turn,
+2 Cards
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While this is in play, when another player plays a duplicate of a card that they already have in play, they first discard a card.
This can't cause a player to discard more than more than one card per duplicate played.
I like to make hard decisions. Torturer is one of my favorite cards. This card does a lot of things. One thing it does is it makes you make decisions. Do I want to play this card if I have to discard? If so, what do I discard? If not, that's a card I can discard, so what card can I play now? This card also encourages deck variety: If you play no duplicates, it doesn't affect you. It hurts both engines and big money. The better your deck is, the more it hurts you. If the game is a slog, then you will have lots of junk cards to discard anyway and few duplicates to play, so it doesn't over-slog games.
So, how powerful is the attack? Well, first note that multiple copies do not stack. Discarding 2 cards per card played would be ridiculous. I estimate that a good engine will play 6-8 duplicates per turn when drawing your whole deck. That's 6-8 cards to discard. That sounds like a lot! Well, that's out of your whole deck. That's equivalent to having 6-8 Curses or other dead cards in your deck, so it's no more powerful than Witch in a four-player game. But you get to choose which cards to discard rather than simply drawing dead cards, so it's not quite as bad. It doesn't punish you until you start playing several cards, so it at least lets you get your turn started. If you have a hand of five coppers, you end up discarding two of them, so it's no more powerful than Militia in the worst case.
So let's review: It forces tough and meaningful decisions; it isn't oppressively powerful but still very effective; it scales with quality of your deck, affecting better decks more; encourages deck variety; and doesn't worsen slogs (or if it does, not by much.)