It sounds like a tapeworm Rachel.
You can worm ducks, like you worm chickens, by mixing Flubenvet in with their feed for a week. Marriages make pellets already mixed which is way easier, you get the dose and mixing right.
It's usual to do them every 6 months because they certainly can pick worms up (roundworms too) from wild birds and also slugs, snails, beetles etc. can carry the intermediate stages of some worms.