Doppelgänger

Doppelgängers were individuals who were identical to other individuals. According to the Second Doctor, the idea of a doppelgänger could be found in folklore and myth across almost every civilisation. It was suggested that if two doppelgängers met, one of them would die. (AUDIO: The Night Witches)

Coincidental doppelgängers
Ian Chesterton and Robin Hood happened to look alike, (PROSE: The Thief of Sherwood) as did Polly Wright and Maid Marian, (PROSE: The Thief of Sherwood, That Time I Nearly Destroyed the World Whilst Looking for a Dress) and Tatiana Kregki, (AUDIO: The Night Witches) Strella and Romana I, (TV: The Androids of Tara) Ann Talbot and Nyssa, (TV: Black Orchid) Peri and Queen Anne (although Queen Anne had a crooked nose), (AUDIO: The Church and the Crown) William Pike and his great-grandson Darius Pike, (TV: The Cambridge Spy) Barker and his great-grandson Thorne, (TV: The Cambridge Spy) Gwyneth and Gwen Cooper, potentially due to spatial genetic multiplicity, (TV: Journey's End) cousins Adeola Oshodi and Martha Jones, (TV: Smith and Jones) and a Soothsayer and Amy Pond, (TV: The Fires of Pompeii)

The Doctor's doppelgängers
The Doctor had many doppelgängers. This included the Abbot of Amboise, Robert Bellarmine, Solomon the trader and the First Doctor, (TV: The Massacre, PROSE: The Empire of Glass, TV: Dinosaurs on a Spaceship, Twice Upon a Time), Ramón Salamander and the Second Doctor, (TV: The Enemy of the World) Premier Lutz and the Third Doctor, (PROSE: Talons of Terror) Andrew Edwardson and the Fifth Doctor, (AUDIO: My Dinner with Andrew) Commander Maxil and the Sixth Doctor, (TV: Arc of Infinity, The Twin Dilemma) Lobus Caecilius, John Frobisher and the Twelfth Doctor (TV: The Fires of Pompeii, Children of Earth, The Girl Who Died)

Deliberate doppelgängers
The Daleks created a robot double of the First Doctor, (TV: The Chase) the Chameleons adopted the appearance of several people including Polly, (TV:The Faceless Ones) General Scobie came face-to-face with his Auton replacement, (TV: Spearhead from Space) Bill Filer was duplicated by Axos, (TV: The Claws of Axos) the Zygons impersonated several people including Harry Sullivan, (TV: Terror of the Zygons) the Fourth Doctor, Sarah Jane Smith, Sergeant Benton, and Harry Sullivan were all duplicated as robots by the Kraals, (TV: The Android Invasion) Prince Reynart's robot double was crowned king in his place, (TV: The Androids of Tara) Cessair of Diplos created an image of the Doctor that caused Romana to fall over the edge of a cliff, (TV: The Stones of Blood) Romana II chose to take on the appearance of Astra of Atrios when she regenerated, (TV: The Armageddon Factor and Destiny of the Daleks) Meglos and the Fourth Doctor, (TV: Meglos) Omega and the Fifth Doctor, (TV: Arc of Infinity) Kamelion took on the form of King John of England and momentarily, the Fifth Doctor, and Tegan Jovanka, (TV: The King's Demons) the Daleks created multiple duplicates, Stien amongst them, (TV: Resurrection of the Daleks) Sharaz Jek created android doubles of Major Salateen, the Doctor and Peri, (TV: The Caves of Androzani) a Kymbra Chimera took on the form of the Sixth Doctor and Peri Brown while in the Doctor's TARDIS, (COMIC: Changes) Dr. Who and the Seventh Doctor, (PROSE: Conundrum, Head Games) a clone of the Sixth Doctor, (AUDIO: Project: Lazarus) the Autons created a duplicate of Mickey Smith, (TV: Rose) Martha Jones was cloned by the Sontarans, (TV: The Sontaran Stratagem) Drake created a duplicate of K9, (TV: Jaws of Orthrus) the Autons created a duplicate of Rory Williams, (TV: The Pandorica Opens/The Big Bang) the Flesh was used to create "Gangers" of both the Eleventh Doctor and Amy Pond, among other characters, (TV: The Rebel Flesh/The Almost People and A Good Man Goes to War) the Justice Department Vehicle known as the Teselecta took the form of Grigori Rasputin with green skin, a caretaker, Erich Zimmerman, a Nazi soldier (both with and without a motorbike), Amy Pond, River Song, (TV: Let's Kill Hitler) and the Eleventh Doctor, (TV: The Wedding of River Song) the spirits of the 49th century took bodies on purpose so that they could be free once more (AUDIO: The Spirit Trap) and the Twelfth Doctor and Lobus Caecilius. (TV: The Girl Who Died)

Other
The Third Doctor travelled to a parallel universe and encountered other versions of Alistair Gordon Lethbridge-Stewart, Liz Shaw, and John Benton. (TV: Inferno) The Tenth Doctor travelled to a parallel universe and encountered other versions of Mickey Smith, Jackie Tyler, Pete Tyler, and the Cybermen. (TV: Rise of the Cybermen) Zagreus and the Eighth Doctor were identical. (AUDIO: Neverland, Zagreus) Townsend, Tepesh, and Winky were projections of the Seventh, Sixth and Fifth Doctors. (AUDIO: Zagreus) The Tenth Doctor transferred his regeneration energy to his hand, and when Donna Noble touched it, she created the Meta-Crisis Doctor. (TV: Journey's End)