WOW thats not good. If one does that again, its best to do aggression training with them. Any type of poultry or waterfowl, immediately pick them up when it happens, hold them tightly (not so tight it harms them). I do a fast grab by the neck with one hand, other arm around those wings and body, grabbing the legs. Ducks have big strong wings and it really hurts to get flogged! Carry them around a bit, so they are fully aware you are head duck in this instance. Pinning a bird, which means holding them down to the ground in cases the bird is to large to pick up is another option. Its the natural way the birds show dominance to another bird in the flock. I have the biggest issues with my young ganders and thats how I have always handled it. Also works in cases of large tom turkeys! I learned from my grandmother to watch how your animals act, and use the methods they do themselves to teach them whats acceptable, and whats not.
Yes they do, its not as high a mohawk but they do have the ability. I only see my girls do it if someone bothers her nest, or if I have added new Muscovy to my flock. If a duck has a mohawk she is probably pretty angry or feels the need to be defensive/territorial.