are you sure it's a wild mallard? Domestic mallards are a bit bigger than wild mallards. Someone could have dropped you off a pet or a pet flew in.
Edited to add: its' not that i dont believe a wild mallard can fly in and join up with domestic ducks. But the fact that you didn't notice a difference in the new one other htan noticing you had an extra duck has me wondering
I always hope that my ducks will call in a wild mallard OR someones pet that's flying over...but so far no deal