From what I've heard here on BYC, ducklings that have the crested gene from both parents often have neurological problems. Symptoms might not show up for a couple of weeks. Some people have had success in keeping them from getting too bad by keeping up with good nutrition, but others have had to put their ducklings down. There's really no way to tell if your duckling has two of the genes without being tested until it shows symptoms.
Chances are, your duckling is fine.
I think there's a site that talks about crested ducks somewhere. Someone else might post it.
ETA: Most crested ducklings with 2 of the genes don't make it through the hatch.