Citations:doppelcest

Noun: "(fandom slang) sexual activity with one's doppelgänger"

 * 2013, Maria Lindgren Leavenworth & Malin Isaksson, Fanged Fan Fiction: Variations on Twilight, True Blood and the Vampire Diaries, McFarland & Company (2013), ISBN 9780786470440, unnumbered page:
 * Inventive variations on the intimate stranger theme are found in “doppelcest” fics exploring the physical and supernatural match between the vampire Katherine and her doppelgänger Elena.