This doesn't make any sense. First of all, the Blue Falcon, the Koopa Clown Car, and whatever other things you're going to pull out of your ass, are all inanimate items. They are not actual characters.
But here's the thing: Why is Olimar called "Pikmin and Olimar", and why is Rosalina called "Rosalina and Luma", and why are the Ice Climbers called Ice Climbers and not just Popo or Nana (depending on which one takes the lead)? Because the Pikmin, Luma and other Ice Climber are part of the team that composes the character. Obviously, Olimar and Rosalina are the leaders of those teams, yet the Pikmin and Luma get recognition - why aren't you complaining about them being credited instead?
The hunter is as much part of the team composing the Duck Hunt character as the dog and the duck are, even though they are not physically present on the battlefield. Calling the character a "duo" is ignoring the third, and essential, member of the team. And to top it off, the hunter is a significantly more important member of the Duck Hunt character than the Pikmin, Luma and second Ice Climber are. You can play Solo Olimar, Rosalina and Climber by getting rid of their other team members, but you cannot play Duck Hunt without the hunter, since they are an actual part of the moveset - you can't drop the hunter into a pit like the other characters.