I don't know how much of a health concern it would be outside of the weight gain. Obviously If she's eating enough of it to gain weight she's eating way too much.
Personally, I would only feed the ducks as much as they can at a time. I would probably start by feeding them a couple of times a day and seeing how much they actually eat in a 15-minute period or so and then remove the food. Once you figure out approximately how much they eat just give that to them.