A ditto match or a mirror match is a match where both players choose the same character. They are different from other matches in that neither player has a match-up or stage advantage over the other player. The name comes from the definition of 'ditto', which means 'the same'. Ditto matches occur at times during tournaments.
Strategies
Strategies for a ditto match often vary 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; this variation is removed during a ditto match. Another example would be in a Super Smash Bros. Captain Falcon ditto match, where even low ranked Captain Falcons can take out top people due to incredibly easy combos, or Yoshi ditto matches where DJC'ing isn't as common. A general strategy is to be unpredictable at all times and use your experience with your character (knowing combos or recovery strategies) against the other player. Snake is a good example of this, as using explosives in different ways can make the battle confusing for the other player.