I don't know too much about incubating duck eggs, but I have had a hen sit on eggs for longer than 21 days if no eggs hatch. You might be able to remove the chicken chicks when they hatch and she might keep sitting on the duck eggs until the ducklings hatch. You could also have an incubator ready and move the eggs now or when she stops being broody. If you don't have an incubator, it is pretty easy to make one with a styrofoam cooler, a light bulb with a dimmer switch (to regulate temp), thermometer/hygrometer, and a bowl of water with a sponge for humidity. I have made a few in a pinch and actually get pretty good results (50-75% hatch rate). You could let the hen sit on the eggs for a while longer to let you set up the incubator and get the temp regular. Only thing is that you have to turn the eggs manually 3 times a day.
Hope everything works out!