Might you have a picture? Crested ducks happen..people say you shouldn't breed them..but I would think that one crested bred with a non would give you some crested ducklings and some not..it happens...shouldn't be a big deal...regardless of the hype I have seen plenty of happy well adjusted totally normal crested ducks. We used to live in a larger town than we do now, and in that town there's a big lake and park with a whole lot of ducks...some Pekin, some Mallards, Cayugas..whatever...anyway there was one crested duck that my family affectionately calls afro duck..anyway for 2 years afro duck was there, and this year when we went there there were 4 afro ducks now..so afro duck one has been breeding apparently. They are so cute, and seem to be doing just fine. So for real, don't worry too much..and if you are terribly worried then don't hatch any eggs from this bird. That way the crested thing won't keep popping up.
I also think that crested ducks are crested at hatch...it doesn't just come up one day. It is a gap in the skull that fills with fatty tissue and then the crest grows over that...so if it wasn't crested when you got it, it isn't crested now.