DC's Legends of Tomorrow introduced Marie Antoinette (Courtney Ford), who looks exactly like Nora Darhk, which means there are somehow still doppelgangers on Earth-Prime even after the Arrowverse's Crisis On Infinite Earths. Legends of Tomorrow wrapped up Crisis by introducing the amalgamated Earth that combined the former Earth-1 with Supergirl's Earth-38 and Black Lightning's world. However, Legends of Tomorrow season 5 quickly left the fallout of Crisis to the other series since our time-traveling heroes have their own crisis: rounding up history's greatest villains (dubbed Encores) resurrected from Hell and causing chaos across time.
Ever since The Flash opened up the Multiverse, several Arrowverse characters have met their exact doubles. Before Crisis happened, multiple versions of Harrison Wells (Tom Cavanagh) joining Team Flash became a tradition and Barry Allen (Grant Gustin) has met his Earth-2 doppelganger. Barry also dealt with the confusion of Earth-90's Flash (John Wesley Shipp) looking identical to his late father Henry. Supergirl (Melissa Benoist) has also battled her evil Russian double, Red Daughter. Even after Oliver Queen AKA The Spectre (Stephen Amell) rebirthed the Multiverse during the Crisis, doppelgangers have reappeared on Supergirl, like multiple Brainiacs, but they were from the parallel Earths destroyed during the Crisis. And, of course, during the Crisis, Ray Palmer (Brandon Routh) met his exact double, who happens to be the Superman of Earth-96.
However, as the Arrowverse's renegade outlier series, Legends of Tomorrow looks like it's playing by its own rules post-Crisis. In this week's episode, "A Head of Her Time", the Legends ventured to 1793 Versailles to find Marie Antoinette, who is the exact double of Nora Darhk. The Queen of France was back from Hell and taking vengeance for her beheading by throwing an eternal party that was killing her guests. The Legends apprehended Marie, only to discover she was still beheaded - which meant tying up her headless body while keeping Antoinette's perfectly sentient head in a separate room on the Waverider. It gets even weirder: Ray excitedly showed Nora (the head of) her French doppelganger and suggested she take a 23-and-Me test because they look exactly alike. Understandably, Nora was far more disturbed that her boyfriend was showing her "a gagged, decapitated woman".
This isn't the first time a Legends of Tomorrow character ran into a historical figure who is their doppelganger. Last season, John Constantine (Matt Ryan) met his ancestor King Konstantyne in 55 B.C., but their resemblance was easily explained by them sharing a bloodline (and it was convenient for Matt Ryan to play both characters). However, Nora, who is the magical daughter of Damien Darhk (Neal McDonough) and currently the Fairy Godmother, has not been purported to have a French or Viennese heritage that ties her to Marie Antoinette. Incidentally, Damien also battled his double, in the form of his younger self, in Cold War-era East Berlin during Legends of Tomorrow season 3.
Barring some kind of timeline-altering chicanery on Earth-Prime, the simplest explanation for Marie Antoinette being identical to Nora would be that the Darhks are blood descendants of French royalty, although given their family history, some kind of dark magical influence isn't out of the question either. Regardless, Marie Antoinette being Nora Darhk's exact double doesn't appear to be tied to Crisis and parallel Earths. Luckily, after her head was reunited with her body, Marie Antoinette remains a prisoner on the Waverider along with the remains of Grigori Rasputin. So, it's possible an explanation for how Marie Antoinette looks exactly like Nora Darhk is on the way in a future episode of Legends of Tomorrow when Nora is back on the Waverider and the former Queen of France meets the Fairy God Nora face-to-face.
DC's Legends of Tomorrow airs Tuesdays @ 9pm on The CW.