quickie

grandad son son

haha, tricky.
i need to get better at this riddle stuff.

You could also say that there are only two people total. Two fathers, each who are undoubtedly someone's son. Every father is a son, but not every son is a father.

So the question becomes from where did the extra duck come?

one of the ducks could have been pregnant

I don't think ducks get pregnant...

Well, technically no, they don't, but the duck could be carrying a fertilized egg. Then there would be another duck, even though it's still inside the egg inside the duck.