If you haven't already heard the news, Mark Hamill has officially joined the star-studded cast of The Rise of Skywalker, returning to his iconic post as Luke Skywalker. While it's most likely that Luke will return as a Force spirit, what if he doesn't? The film's first trailer features Hamill in a voiceover, in which he ominously says, "No one's ever really gone." Sure, this could just be plain old wisdom, but it could also be foreshadowing for Luke's return in an unexpected form. In fact, there's a pretty convincing theory floating out there that Luke may be a clone.
Remember, Vader brutally slices off Luke's hand in The Empire Strikes Back. Given that the hand doesn't get destroyed, it would contain prime, cloneable DNA. This twisty idea dates back to Timothy Zahn's 1993 Star Wars novel The Last Command. The Dark Jedi Joruus C'baoth takes the severed hand to create an evil Luke, i.e. "Luuke," who eventually gets defeated by the real Luke. This book isn't canon, but that doesn't mean that filmmakers won't and can't borrow ideas from it. If Luke's lightsaber could survive (as we find out in The Force Awakens), it's likely that the Jedi's hand didn't get destroyed and could have been salvaged for DNA in the sequel trilogy.
As a side note, there's also a strong subset of people who believe that Rey may be Luke's female clone grown from his severed hand after all of this time, which may explain her gift with the Force. Episode IX may not show a duplicate of Luke, but instead, could reveal this secret. Sure, Kylo Ren tells Rey about her parentage in The Last Jedi, but he's not exactly the kind of person that you can trust.
With Rey and Kylo Ren going face to face in The Rise of Skywalker, introducing clones could make things beyond convoluted, suffice to say. If Luke gets cloned, who knows who else could have been cloned? Star Wars has featured so much dismemberment over the years, with Mace Windu and Darth Vader as other prominent examples. The Luke clone theory could yield a fruitful narrative if crafted well, but we'll just have to wait and see how and if it pans out.