For a 4 night cruise, is the restaurant that you dine at twice just randomly assigned?
It's not exactly "random" -- there is a rotation, and every guest on your rotation will go to the same MDR each night along with your assigned serving staff (you'll have the same servers every night). With 3 MDRs obviously 1 will be repeated on a 4-night cruise. I think the Wish rotation repeats on night 3, so the options are:
1923, Marvel, Marvel, Arendelle
Marvel, Arendelle, Arendelle, 1923
Arendelle, 1923, 1923, Marvel
As others have mentioned, the menu will be different for one of the repeat nights. However, I guess it all depends on your
reason for not wanting Marvel twice -- if it's the decor/atmosphere, etc., not having the same menu will be a moot point. You can request a rotation in advance, it's only a request and not guaranteed, but I suggest doing so now rather than waiting until you are onboard. Once onboard they may be very limited in what changes can be made based on how full is that cruise.
Alternatives to the MDR are booking one of the adult-dining venues (extra cost), or the quick-service locations near the pool (check the hours), or ordering room service.