Well, if you have a bare neck or open spot on the back of your duck... you have an issue! There are 2 ways to fix it!
Get more females or lose a few males.
You see, there is what people who own ducks and they call it "duck math" and at some point or another we all fall victim to it. How it works is that one male should have at LEAST 2 female companions. The more female's to male ratio the better. When you have too many males to females the males can hurt your females. Some males may even gang up on one female and really hurt her! If they are in the water they can down her. Males can and will have a "favorite" female that they all want so they get what they want. They will pile on her and if she can't get away they will pull feathers out and draw blood. Ducks do have claws, some people may not know that, and they will gouge those nails right into her back. Ouch!!! Some places will sell "saddles" for chickens but do go up to a size for a small turkey. They help for after a male has over loved his companion with the back but not with the neck so once you noticed something is up the best thing to do would be to separate her until she is no longer raw.You can spray something on her like Blu Kote to help if there is any open wounds.
Here is a great site to find the saddles and other duck apparel!
You can find Blu Kote at a Tractor Supply or other livestock store near you!