Was trying out a new second hand lens at the local pond today and happened to catch a duck swimming around the pond, with a duck on its back, and that duck had a duck on its own back as well. I dont know how long they were like this before I saw them but I watched them for maybe a minute or two before they all split up.
The tail feathers of the one on top suggest it might be female, and they look young, so this might be teenage experimentation.