How long is the incubation period for a duck?
It's 28 days for all domestic ducks except Muscovies, who are 35 days. Muscovies, at least, sit undisturbed for 24 hours when commencing the sit.
If you have "other duck" eggs that you want to add to the Muscovy's clutch, wait a week and make sure the eggs are warm when you slip them under. You have a fair chance of success. She won't mind that one of her ducklings looks a little different from the others.
It isn't diligence: it's the fact that the duck eggs need more humidity so it kind of forces mum to get up, go swimming, get a bit wet so as to transfer humidity to her eggs.
Also, ducks will remove bad eggs (or unfertilized) from their nests and carry them away so pests don't bother the nest. This is normal, but be careful where you walk because ducks leave them all over the place. They never pick a spot and just leave all of them in the same place! This is probably instinctual to help thwart predators, though. They are not being vindictive. (Although with ducks, it's sometimes hard to tell!)
(The term for eggs is incubation. Gestation is for live young, as in most mammals.)