If it would be against another fellow I would like it, but against the CPU I think it is a major letdown. Basically because it is like saying, “it was so hard for us to design another or some more additional characters”. If the game pretends to have a storyline how could you fight against yourself? This is so weird.
However some games create some stories or some arguments for your twin character, then is quite ok and even fun. What I do not like is when the twin character is used to avoid creating an additional character.
If, for instance, in a KOF there are supposed to be 8 matches, and there are 10 teams, I think it is very disappointing that one match should be against your teams.