I can't imagine suffering a glut of duck fat when there are potatoes to be roasted and confits to be made, but if you find yourself in such a pickle, do like Grandmere Yvonne and make some:

Duck Fat Soap  

1 can of Red Devil lye from the hardware store
5 – 6 lbs. of duck fat
5 c. cold water

Line a few shoeboxes with parchment paper, cheesecloth or muslin.

Pour the water into a very large stainless steel or enamel pot. CAREFULLY—lye burns if it gets on you—pour the lye into the water. It will bubble and steam and get very, very hot (like when it's eating through drain clogs). Let it cool to room temperature, 75 degrees or so. DON'T test it with your finger or a thermometer; just feel the outside of the bowl.

Meanwhile, melt the duck fat and let it cool to body temperature.

Put on some gardening gloves and, with a long-handled wooden spoon or wire whisk—CAREFULLY, so it doesn't splatter—add the fat SLOWLY to the lye mixture. Keep stirring, until it begins to thicken like custard. This may take as long as 20 minutes, or it might happen right away.

Pour the mixture into the shoeboxes, and cover them loosely with a cloth. In a day or two, they will be hard. Turn them out of the boxes, and cut into bars. Keep them somewhere cool and dry and out of reach for 3 to 4 weeks to cure, after which the lye will have been eaten by the fat and the soap will be gentle enough to wash a baby.