The DC TV Universe has massively expanded since Arrow first came on the air with its basic story of 'street vigilante vs criminals.' Not only do we now have so many more characters (many with fantastical powers) thanks to Flash, Legends, and Supergirl - we also have multiple versions of those characters from throughout the multiverse.
Supergirl's setting on an alternate Earth (Earth 38 to be specific) already placed it within the DC TV Multiverse - yet we haven't run into dopplegangers of DC TV characters from Earth's 1 and 2. That's something of interesting flaw in continuity, so will it be something that Supergirl will address?
According to executive producer Andrew Kriesberg (via TV Line): it won't be. According to Kriesberg, “the problem with that is if we do that once, we’re going to open the floodgates of insanity."
There's a lot of truth in that statement: alternate Earth dopplegangers already make the characters of Flash and Arrow harder to keep straight - not to mention Legends plucking characters from different time periods, as well. Behind the scenes, getting actors to play different versions of their characters on the same show is easy enough; scheduling things so that actors can play alternate versions of their characters on different shows is a whole other beast.
According to Kriesberg, it would take “a really good story” for Supergirl to get alternate versions of Barry or Oliver on the show; or conversly, an alternate version of Kara on Earth 1. In short: it would have to be an event. Good thing we'll be getting said event!
Distributed worldwide by Warner Bros. International Television Distribution, SUPERGIRL is executive produced by Greg Berlanti (Arrow, The Flash), Ali Adler (The New Normal), Andrew Kreisberg (The Flash, Arrow) and Sarah Schechter (Blindspot, The Mysteries of Laura). Supergirl is based on the characters created by Jerry Siegel and Joe Shuster and appearing in DC Comics, by special arrangement with the Jerry Siegel Family.
"Homecoming" episode of Supergirl airs Monday, February 27 at 8 p.m. ET/PT.