"How would you feel if you met another you from a parallel universe? One may well exist, as a ghostly double or evil twin, ready to crush your sanity or even replace you altogether!"
https://mythology.net/others/concepts/doppelganger/
"A doppelganger is a shadow self that is thought to accompany every person. Traditionally, it is said that only the owner of the doppelganger can see this phantom self and that it can be a harbinger of death. A person's friends or family can sometimes see a doppelganger as well. The word is derived from the German term for "double walker.""
https://www.liveabout.com/true-stories-of-doppelgangers-2593448
"Somewhere in the unfathomable span of space, there exists a replica of this planet, a replica of the continents, a replica of the countries and cities, and a replica of you, an alternative you."
https://www.anaedoonline.ng/2021/09/28/alternative-earth/
"So any chunk of matter can also occupy two places at once. Physicists call this phenomenon "quantum superposition," and for decades, they have demonstrated it using small particles."
https://www.livescience.com/2000-atoms-in-two-places-at-once.html