I just finished watching a Netflix series (which I won't name because there are spoilers ahead) at the climax of which the hero learned the only way to avert apocalyptic disaster was for him to go back in time and refrain from a certain action he performed at the beginning of the series. Thereby, everything he'd done since then would never have happened. And of course nobody he'd come to care for over the course of the series would remember meeting him and participating in those adventures, because they never happened. The hero asks to be allowed to remember the now-nonexistent events, a petition the sorcerer performing the spell grants. The mage also grants a similar request from the hero's love interest. In the final scene, shortly after the hero has made the sacrifice of finding himself back at the start and choosing not to do what he did the first time around, the heroine joins him. They ride off into the sunset for a life of adventure together. Though the ending is bittersweet (everybody else has still forgotten the hero and his exploits among them), I liked it very much.
However—because we don't witness the conversation between the heroine and the sorcerer, we don't know whether she simply left her home and neighbors (with no explanation, since their memories have been reset) to meet the hero when she knew he'd show up or whether she, too, was magically sent back to the restart point. If the latter, now she is living in two places at the same time, in her home town and on the road with the hero.
Granted, that's not an uncommon situation in time-travel fiction. In the Harry Potter series, Harry and Hermione see their earlier selves when they revisit past events through the use of a time-turner. In THE TIME TRAVELER'S WIFE, the hero often has duplicate selves in existence at the same moment. Heinlein frequently allows more than one of the same person to exist at the same time, e.g. in THE DOOR INTO SUMMER, TIME ENOUGH FOR LOVE, and the iconic short stories "By His Bootstraps" and "All You Zombies."
In strict science fiction terms, though, that phenomenon amounts to having matter (the atoms and molecules making up the character's body) created out of nothing. If two iterations of one person exist simultaneously, where does the material for the duplicate come from? Dean Koontz's novel LIGHTNING postulates that a traveler can never occupy a point in time where he already exists, a rule that not only respects the laws of physics but creates suspense at the climax, when the time traveler has a very tight window in which to save the heroine without bumping into his former self. (That's a fantastic SF romance, by the way, although it isn't marketed as such.)
In the recent season finale of THE LIBRARIANS, a time reset similar to the conclusion of that Netflix series saves the world from a colorless dystopia in which the Library, and therefore curiosity and imagination, don't exist. Since the dystopic timeline constitutes a self-contained alternate world, when it's wiped out by the reset there's no problem of people duplicating themselves. In the case of such works as THE LIBRARIANS, the Harry Potter novels, and the Netflix series, we can say it works because it's magic. In SF terms, the unfinished story "The Dark Tower," attributed to C. S. Lewis (some scholars have doubts about the authorship), carries the paradox to the logical conclusion by declaring that physical time travel is impossible, because in either the past or the future the atoms making up the traveler's body would be dispersed elsewhere throughout the environment. A character in the story invents a device for remotely viewing a different time period, though. The protagonist of a short story whose title and author I don't remember discovers that, while physical time travel is impossible, he can project his consciousness into the minds of other people in the past. He uses the technique to invade Hitler's mind—and, not surprisingly, incites the global tragedy he's trying to prevent. (TV Tropes calls this phenomenon "Hitler's Time Travel Exemption." Anything a time traveler does to try to thwart him will fail or even produce a worse outcome.)
In my opinion, allowing corporeal time travel makes for more interesting fictional scenarios, even if they have to be justified with, "It's magic."
Margaret L. Carter/p> Carter's Crypt