I've read of geese hatching other species. I personally wouldn't let her set on any eggs but her own, or another goose.
When mine start to lay they often will spend a lot of time laying, and tending the nest only to walk away at night. So they can fool you into thinking they are ready to set...
Goose eggs seem to always be significantly larger than duck eggs. It could be hers or the ducks. We are approaching breeding season so she may either be laying or feeling motherly. Time should tell.