Perfectly normal albeit annoying behavior. Some birds aren't very discerning when it comes to figuring out which eggs are theirs. (One of my ducks was sitting on eggs when she wasn't even laying for the season yet!) Once they go broody and start sitting on eggs, they will stop laying eggs.
Make sure there aren't any eggs for her to sit on, and your mallard will eventually start laying again once her hormones switch back to "laying mode." It may take a few weeks for her to start laying again depending on how quickly she gives up on trying to sit on eggs.
