Ganger

Gangers, shortened from "Doppelgängers", were a clone race created by humans from an artificially created organic substance called the Flesh.

Biology
Gangers were mimicked duplicates of individual humans who controlled their Gangers through telepathic remote control via a specially-designed harness. This allowed humans to work in extremely hostile environments without risk to themselves, as the Gangers could be easily recreated and any damage they sustained would not be felt by the real human as the nerve endings automatically shut down. (TV: The Rebel Flesh)

In at least one case Gangers exhibited the ability to extend their limbs and neck, due to the unstable molecules of the Flesh material. (TV: The Almost People)

The Gangers were mostly unstable and shift between half-Flesh and full human faces which was said to be hard to maintain, at least in the early stages. (TV: The Rebel Flesh) By the 52nd century, this appeared to have been corrected, with Amy Pond's Ganger being indistinguishable from humans for months at a time. (TV: Day of the Moon, A Good Man Goes to War, et al) Exposure to the inside of a TARDIS stabilised a less advanced Ganger and made them fully human. (TV: The Almost People)

History
Throughout the 22nd century, the Ganger process became common use at many industrial facilities and by the military. It was believed that there were ten million Gangers in India alone. (TV: The Rebel Flesh / The Almost People)

A Ganger was electrocuted, in an incident at the Isle of Sheppey, resulting in the Ganger gaining sentience and horrifically killing its operator.

A similar event to Sheppey occurred at St John's Monastery, an acid-mining facility that used Gangers. After the facility was hit by a solar storm, the active Gangers gained independent thought and motion, being implanted with the memories of their templates and becoming increasingly fixated on the possibility of making their own lives. (TV: The Rebel Flesh) This possibility became a reality for the Ganger of Jimmy Wicks after the death of the real Wicks. Miranda Cleaves and the Ganger of Dicken, who also replaced his original, revealed the event to the press and fought for Gangers' rights. (TV: The Almost People)

In the 52nd century, the technology still existed. The Church and the Silence created a Flesh duplicate of Amy Pond without her knowledge, and were able to create a complex link that projected Amy's mind into the Ganger no matter where the TARDIS travelled. A Ganger of Melody Pond was also created to fool Amy and the Eleventh Doctor. (TV: The Almost People, A Good Man Goes to War)

The Twelfth Doctor created a Ganger of Clara Oswald for one of her birthday parties. (COMIC: The Partying of the Ways)

Behind the scenes

 * The Brilliant Book 2012, a book that contains non-narrative based information, exhibited further information about Gangers:
 * The Ganger process was developed in 2120 by the Bio-Cellular Institute of Dungeness.
 * Phobias about Gangers spread, with groups like the "Fear the Flesh!" hyperweblog.
 * India was one of the first nations to grant Ganger independence. Pressure would lead to Gangers elsewhere gaining independence: people would later marry the Gangers of their deceased spouses and a sitcom called I Live Next Door was created about them (and co-starring a Ganger of the lead actress).