I have a Muscovy with angel wing. I got her that way. And I have two others without it. I believe it happens due to poor nutrition as a duckling. Someone on here will know. Anyway, if it is caught early enough the wings can be wrapped and it can be corrected. I don't know if the parent has it, if it is likely it's ducklings will.
My girl gets around fine, she just looks funny. We keep her wings clipped even though she cannot fly with them like that because she can't get through the little hatch door in their shed if they spread too wide. But other than that, there is no issue with it.
I have also heard it referred to as "Airplane Wing".