My duck laid an egg!!

Tonightw hen I wend out to feed I noticed an egg in the duck run. It was frozen and broken, but it was an EGG! I found another inside their house and brought it in. I I gave it a bit of a spin and it went around pretty well so I am thinking it froze also. So now I need to know a couple of things: I got both of them as ducklings last year from Hidden River and assume they are brother/sister, if their eggs hatch will the babies have problems because of this? If not, is it true that ducks need to be in water to mate? All they have right now is a heated dog dish. How long will she continue to lay for?

Some of my ducks are just starting to lay now as well, and an occational turkey egg.
Most ducks prefer water to breed in but it is not necessary for bantam or smaller breed ducks, mostly for very heavy breeds.
As for brother and sister it shouldnt cause major issues.