I agree, your brown duck looks like a khaki Campbell, but your duckling looks mallard (even the size says mallard). Are there any wild mallards around who could have snuck a egg into your coop? Or, it could be a throwback from your khaki, but I'm not sure if that happens with that breed. I've never heard of throwbacks with Pekins.
.. in my location we dont have many wild ducks and nobody else in town has ducks or chickens
