And how did they know it was a skunk? I've never heard of this, or seen it. And a skunk going after a duck? Wow. Maybe you have mutant skunks! Or ours are mini-versions. LOL Generally, skunks would rather go after small fare.
Our skunks go after bugs and worms, dog and catfood are favorites too. They are omnivores, but not something that would go after such a relatively large animal. Maybe mice also, kittens maybe. They are not fighting animals, although they can if required.
Edited to add: a weasel animal might do this, or a raccoon maybe as they will go for the neck. Weasels will kill and kill and kill, then just eat what they want and leave the rest.