In an interview with Polygon, Peele mentioned that some of the inspiration for Us came from a 1960 Twilight Zone episode called "Mirror Image," which follows a woman being haunted by an identical version of herself.
"There's something about this idea that the doppelganger that has this creepy smile . . . they know more than you know," Peele explained. "I was sort of connecting that to, first and foremost, our fear — our societal fear — of terrorism, of an attack, of an invader coming in who has been plotting something mysterious. Besides the fact that this is an awful event is the idea that there is a well-oiled plan. And the only other thing that's more terrifying than that is the suppressed feelings of what our part in these tragedies is, even if we are the victim."
The "creepy smile" aspect of the doppelganger fear that Peele mentions is certainly present in Zora's double, Umbrae, who even Red describes as a "monster."