That doesn't matter. Some people like to complete it for shits and giggles, not just so they can get a thumbs up from Oak.
Besides if you are going to make a fuss over there being more than one Mewtwo, then they may as well only make one copy of each game, otherwise you have thousands of the same person going on an adventure.
Point is is that if you get critical over "Theres only meant to be one!" then you are overlooking that there are numerous things that there should only be one of. Theres no point in trying to over complicate things over it.
Besides if you are going to make a fuss over there being more than one Mewtwo, then they may as well only make one copy of each game, otherwise you have thousands of the same person going on an adventure.
Point is is that if you get critical over "Theres only meant to be one!" then you are overlooking that there are numerous things that there should only be one of. Theres no point in trying to over complicate things over it.