My brother can't stand ditto matches in fighting games. Ditto matches are matches when both players play the same character. Over the years he has described ditto matches as boring and uninteresting. For the longest time I couldn't figure out what irked him so about two players using the same character.
Last year at a local tournament for Super Smash Brothers Brawl, we were watching a tournament match featuring two Ikes clashing swords. One of my friends, who was also watching the match, made a few observations about a behavioral pattern common in ditto matches. My friend described a ditto match as being a battle of "monkey see. monkey do." Because both players are using the same character, when one player attempts something, the other player is highly likely to attempt the same thing. This goes back and forth the whole match with each player trying to out do the other at the thing they're both trying to do. It's a lot like playing a game of HORSE.
This monkey see monkey do behavior pattern is useful for determining who has a better grasp of the character, but the part of ditto matches I think my brothers has an issue with is a matter of contrary motion. When both players choose the same character, their movement, attack, and defensive abilities are pitted against the same abilities. So when one player moves forward at a certain speed the other can move back at that same speed. When one does a special attack. The other can retaliate with the same attack making it somewhat less special. Any unique facets of a characters are in some ways diminished instead of being accented like when two different characters fight.
Don't take my word for it. See for yourself. Below are links to ditto matches from 5 different fighting games. Even if you don't understand how each game is played on a competitive level, you should be able to clearly see the ebb and flow of the match and how this interplay is affected by the duplicated characters. See if you can point out all the "monkey business."
SUPER SMASH BROTHERS 64
SUPER SMASH BROTHERS MELEE
SUPER SMASH BROTHERS BRAWL
STREET FIGHTER 4
SOUL CALIBUR 4