A Ditto Match is a match where both players choose the same character.
Ditto matches are used to remove any natural discrepancies that arise between different characters so that skill is the only determining factor in the outcome of a match.
Strategies
Strategies for a ditto match often varies greatly from the character's normal strategy. This is especially the case for characters such as Marth or Jigglypuff whose strategies rely on an advantage in range, mobility or attack speed that is compromised during a ditto match.