This! I've seen ducks and chickens with parasitic twins like this survive. I'd separate the duckling from the others and place it on paper towels so that you can track the bowel movements. The ones that don't survive are usually because their organs are messed up and they can't defecate properly, or they were in the egg too long and just hatch out too weak. If the duckling is relatively mobile, eating and drinking, and defecating normally, there's a good chance it will survive without issue.Can she get around and eat/drink okay?