Not to be confused with Ditto the Pokémon.
A ditto match or a mirror match (often shortened to just ditto or mirror), is a match where both players choose the same character. They are different from other matches in that neither player has an objective matchup or stage advantage over the other player. The name comes from the definition of 'ditto', which means 'the same'. Ditto matches occur frequently during tournaments, often between top-tiered characters, as they are the most commonly played.
Strategies for a ditto often vary greatly from the character's normal strategy. An example would be a Melee Jigglypuff ditto; both players have access to Jigglypuff's strong aerial drift and large, disjointed hitboxes, and neither can be comboed very easily due to Jigglypuff's floatiness. If one player attempts to land Rest to close stocks out early, the other can easily retaliate with their own and force the game back to a neutral position at worst. Due to this, Jigglypuff dittos disproportionately revolve around the neutral game, with the player that has superior patience and spacing usually edging out their opponent. Another example would be in a Super Smash Bros. Captain Falcon ditto match; due to Captain Falcon's incredible combo ability along with his own vulnerability to combos, the match ends up taking advantage of player flaws more frequently than usual.
A general strategy is to be unpredictable at all times and use experience with a character (knowing combos or recovery strategies) against the other player. Snake is a good example of this; in a mirror match of the two, the players have to use projectiles in such ways to make the stage difficult to navigate for the other player.
Ditto matches would appear to always be perfectly even matchups, allowing a 50/50 chance of winning between two evenly-skilled players. However, due to port priority, this is not always precisely the case. The most obvious imbalance regards simultaneous KOs and is overridden in tournaments by the "Suicide Rule". The remaining differences generally have little effect on practical tournament gameplay, though for some characters like Snake the imbalance can be noticeable.