If we regularly saw Kara interacting with Barry Allen or Oliver Queen or Sara Lance, some of the crossover magic might be lost, even if they were simply Earth-38 versions of the characters.
We’ve seen Kara interacting with some of our familiar Earth-1 superheroes off and on during the first two seasons of Supergirl, but the heroes really only join forces for big crossover events.
Andrew Kreisberg’s reveal that the Supergirl team is avoiding doppelgangers to keep from spiraling out of control indicates that we can count on the show keeping its focus on Kara and Co. After all, there’s already plenty of wackiness on Supergirl without bringing in different versions of Barry and Oliver.
We have shape-shifting Martians, a Daxamite with a dark past, and a heroine who can be turned evil when exposed to a certain kind of Kryptonite.