Luke Skywalker's appearance in Star Wars: The Mandalorian was one of Season 2's best surprises, and the goings-on of the Jedi Master will be pivotal going forward as fans will want to know how Grogu's training is progressing. However, there's still a question as to whether the Luke who took Grogu away is the genuine article.
An evil Luke Skywalker clone has been a part of Star Wars canon before, meaning there's a possibility that the creative team of The Mandalroain faked everyone out by sending the galaxy's most precious Force user off with a doppelganger.
Being created from Luke's hand after it was severed by Darth Vader during their battle in Cloud City, the clone, Luuke, appeared in the popular Thrawn Trilogy He was a henchman for another Jedi clone, Joruus C'baoth, serving as an extension of his master's will. Luuke would go on to fight the real Luke to a standstill, but eventually, Mara Jade was able to distract the clone and to slay him. Fighting a dark reflection of himself and seeing him die was a traumatic experience for Luke, one that led him to the Dark Side.
Even after the elimination of Legends, clones remain a major part of the current Star Wars canon. Famously, Emperor Palpatine returned from the dead in The Rise of Skywalker using a clone body, one that allowed him to continue to wield his Force abilities. The Mandalorian has also dealt with its fair share of clones. With Boba Fett returning in Season 2, Din Djarin was fighting alongside a clone of Jango Fett for several episodes.
With that in mind, it wouldn't be a huge stretch to believe that a clone of Luke exists in this continuity. There's a precedent for little to no explanation being needed for a clone's appearance, and Luuke's origin could be recycled because the real Luke's hand has never been recovered in the current canon. However, the likelihood that the Luke that appeared in The Mandalorian is a clone is low. Grogu would have sensed that the Jedi wasn't who he appeared to be when they came into contact and R2-D2 wouldn't have been accompanying him, either, unless there are also evil droid clones in play, too.
Now, while those facts make it clear that the creative team behind The Mandalorian didn't intend for Luke to be a clone, that doesn't mean that the creative team at Lucasfilm won't ever go to that well, especially given that its immediate future lies in Disney+ spin-offs and not movies. A Disney+ series could explore the idea of a Luke clone operating in the same post-original trilogy space The Mandalorian does, interacting with characters like Ahsoka Tano, which would subvert the expectations of their long-anticipated first meeting.
In that sense, an evil Luke has some storytelling potential. However, it's probably for the best that the current Star Wars canon stays away from the crazier side of the EU. Obviously, it would be an interesting idea to explore in something akin to Star Wars' answer to Marvel's What If?, but not as part of the franchise's canon future.