In the season two episode "The Attic," Enver Gjokaj's identical twin brother Demir Gjokaj plays a alternate version of Enver's character who fights "himself" while trapped in a dream. This was actually the second time in his showrunning career that Joss Whedon cast the non-acting identical twin brother of one of his series regulars as a supernatural double or doppelgänger of the regular character; the first time was during "Buffy the Vampire Slayer," when Nicholas Brendon's twin, Kelly, played a part of Xander's personality that had become mystically separated from the original Xander.
Hello, Echo. How are you feeling?
Echo: Did I fall asleep?
Topher Brink: For a little while.
Echo: Shall I go now?
Topher Brink: If you'd like.