A: When something which is obviously completely redundant comes up, for example, if Draw 2 is in play, and someone plays Draw 2 again, the old one is discarded, and the new one replaces it, and game conditions do not change. If Wormhole is in play, and someone plays Wormhole again, no, you don’t get a double-Wormhole, you just replace the old one with the new, and there is no functional change to the gameplay. When Double Agenda is replaced by Double Agenda, the same two Goals stay in play.

(You could just put the new one directly in the discard if you want, but don’t just stack them! Rules which have been superseded should always go in the discard pile so they can be referenced, reshuffled, etc.)

Functionally-redundant New Rule cards, which is to say, cards which have different names, but do the same (or almost the same) thing should be treated the same way. Examples would be Mystery Play, Eureka!, and Open the Door, which are all identical in function to Wormhole.

See our Fluxx Card Comparison chart to see which cards we consider functionally (or practically) redundant. Just click to highlight a whole row, and you can scroll to the right to see all of the cards that do that thing, even if they have different names.

A few non-obvious points would be:
Q: Do Double Agenda and Triple Agenda replace each other as rules? Yes. Double Agenda contradicts Triple Agenda and vice versa, since they’re both determinants of the same parameter: the number of simultaneous Goals allowed.

Bonuses generally stack, however, since they are almost always granted based on conditions which are completely independent from each other. For example the Rich and Poor Bonus do not replace each other, and neither contradicts Party Bonus. Furthermore, although we’ve put the Party Bonus and the Love Bonus on the same line in the comparison chart, we would not consider them to be mutually exclusive, since they grant their Bonuses based on the presence of completely different cards.

Of course, if someone played a duplicate Party Bonus (based on the presence of The Party specifically), it would simply replace the Party Bonus already in play, as described above.