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The first rule states "Your game must redesign some existing game idea" with the example being remaking Portal. So it does sound like you are forced. You could interpret it as redesigning the main game mechanics and not the actual game, though, since it only says that you should redesign an existing game idea, not an existing game. Can still be problematic but way less problematic than making an obvious copy of an IP. Even if you had to, since there's no price, you don't have anything to lose by submitting a game that doesn't follow the rules. 

So yes, copying an IP is definitely problematic copyright wise. There are probably many nuances to this, though, and I doubt there's a lawyer here to properly answer that question.