In your new member post, you mentioned your son takes her down to the river every day where she likely finds a lot of earthworms and slugs to eat- they are intermediate hosts for tapeworms - so it is likely that’s where she gets them from.
How many days has it been now since her second dose of the dewormer your vet gave you - I know you said you gave her a second dose after 14 days.
More questions
What is her feed?
Did you also deworm your other Ducks? They can keep catching worms back from each other‘s poops.
Some other things you can do to strengthen her immune system give her some good poultry vitamins, like roosterbooster poultry cell, or nutridrench .