…for example, if we wanted to mix two Batman Fluxx decks and there are two BatComputers in play… or even just putting in multiples of a promo card, like the Ugly Sweater. And how should duplicate New Rules be handled?
A: Well it might seem like a basic question but I don’t think we’ve ever really answered it before. The truth is, we prefer each Fluxx as a standalone experience and don’t really encourage combining decks. We certainly don’t recommend combining multiple copies of the same deck. In almost all cases, each card in a Fluxx deck is unique. Mixing two Batman decks together creates numerous illogical situations. Sure, there’s the question you’ve asked directly, about two BatComputers, but there will also be two Batmans, two Bruce Waynes, etc. Given that you have to discard Bruce if Batman shows up, then it would seem impossible to have two Batmans in play at once as well. And so on. The deck becomes too big to shuffle as well. We just can’t see a good reason to combine two of the same deck — if you want to play with a big crowd, you’re better off splitting into two smaller groups each playing their own game with their own deck.
That said: Nothing is forbidden. Fluxx is a very accommodating system. There’s no rule against having two copies of the same Keeper, unless you add one as a house rule. The Batman/Bruce Wayne restriction is one example of such a rule. So, unless there’s a New Rule or other fine print requiring some action, redundant Keepers can happily co-exist. As for ability stacking, again, barring a New (or Meta) Rule that provides restrictions, each redundant Keeper would have its full ability.
Regarding duplicate New Rules in a game, we would think that multiple copies of the same New Rule would be considered to simply replace themselves on the table, so that if Draw 2 is out, and someone plays Draw 2, it would just replace it’s copy. Though they don’t contradict each other, they shouldn’t stack, since we’d certainly rule that one could only have one Draw rule in play at a time.
Same with other rules which might imply stacking. For example, the Mystery Play rule says you can only take that free action once per turn, so we would rule that you can’t have two of them out, and get to do that twice per turn, since using the second copy would be disallowed by the first copy (and vice versa). In that sense they are kind of contradictory, or, since you could only use the function of one copy, even if there were two in play, they could be considered to simply be redundant.
So you could choose to reduce the New Rules so that there was only one of each in play… but this might unbalance the deck (see how complicated this gets? Just one reason we don’t recommend mixing them!)