It is a bad idea to leave food and water near a nest. It attracts predators and pests. If one or more of your various hens does go broody on the nest, it needs to get off of the nest and move around.
It is common for guineas to use a communal nest. In your case they considered your duck nest a communal nest.
I had one guinea hen that would snuggle up next to a broody turkey hen to lay her egg.