If Lucy stays broody she will hatch and raise the ducklings. However, if the momma duck is also broody you might try getting some of the eggs out from under Lucy, make her a nest someplace that is isolated from Lucy, introduce her to it and may need to restrict her movement from that nest until she accepts it. Otherwise, just keep her away from Lucy if necessary so that she does not challenge Lucy on the nest. Geese can be pretty aggressive when protecting their nest.
The other option would be to remove Lucy and isolate her which sounds like it might be harder to do.