Mallards do not have two eye stripes. They have one. If you got it from a tractor supply it's most likely a Rouen. Especially if it was in an assorted duck shipment. If it is also large that is yet another sign it's a Rouen. Also, check its feet. Is it missing a back toe? If not then definitely a Rouen. Domestic bred Mallards have to be marked so when or if they fly away they don't get confused with wild mallards. Rouens look almost the same as mallards in feather color and pattern. They can't fly though (at least not far) so you won't lose a pet because it wants to fly away. My Rouens look just like yours. And they are only around 6 weeks old and the size of a mallard.