The Mirror Match is a special match battle introduced in Mortal Kombat. The player has to face the character that they're playing as in kombat. The difference between each identical character is distinguished by their clothes.
In the original Mortal Kombat tournament, kombatants had to not only face their enemies in battle, but themselves. Facing a doppelganger, players had to defeat a warrior who shared their own strengths and weaknesses. Essentially, it is a duel where one must overcome themselves if they are to face the likes of Shang Tsung as they become truly aware of their own strengths as well as the weaknesses they can conquer.
In Mortal Kombat X, mirror matches are a topic of interest thanks to the unique interactions that each kombatant has before a fight. In one instance with Reptile, his double is Shang Tsung himself when Reptile assumed he was Chameleon. Other instances heavily imply the convergence of different timelines as Raiden must fight against an alternate version of himself who fights for Chaosrealm instead of Earthrealm while the Scorpion of the new timeline is faced against his original counterpart during his time as the Elder Gods' champion, with Lt. Sonya Blade meeting her future Gen. Sonya Blade self. Also, Shinnok faces his double who [he] "didn't kill at the pyramid".