Warning: contains spoilers for Immortal X-Men #4!
A popular X-Men villain is about to get his very own Spider-Man Clone Saga - except the villain of the storyline is already known before it begins. The conniving villain Mr. Sinister is the foremost expert on cloning when it comes to villains in the X-Men universe, utilizing his expertise to push the boundaries of science and further his goals for conquest and odd experimentation on genetics. Now in Immortal X-Men #4, Sinister's technology has finally got the better of him: there is now more than one of him.
The Spider-Man Clone Saga is infamous in the Marvel community for turning an inspired idea into a slog of a story. Ben Reilly - Peter's clone from a one-off story in the 70s - returns and is eventually revealed to be the true Spider-Man, while Peter is revealed to be a clone. The story progressed through numerous twists, turns, and one plot hole after another; both readers and writers wanted the narrative to end, but Marvel editorial turned a six-month story into a bloated two-year affair. By the time the story ended, Peter was reinstated as the original Spider-Man and the clone died a heroic death (though he eventually returned in 2017's Dead No More: The Clone Conspiracy).
In Immortal X-Men #4, written by Kieron Gillen with art by Michele Bandini, a Quiet Council meeting discusses the ramifications of Cyclops' latest enemy: Doctor Stasis, a scientist ostensibly working for the anti-mutant organization Orchis. But Doctor Stasis is actually the real Mr. Sinister - or so he claims - and the Sinister on the Council is a clone. Sinister is genuinely surprised - but that doesn't stop him from having fun on the X-Men's behalf, escaping and running from the Council before eventually being captured. After a stint in his genetics laboratory, he's kidnapped in a beam of light, disappearing from Krakoa entirely.
In the original Clone Saga, Spider-Man's enemy was the Jackal, a scientist who created multiple clones of Spider-Man including Ben Reilly and Spidercide. But in Mr. Sinister's case, he is almost certainly the villain of this piece; his expertise in cloning would make creating duplicates of himself a trivial matter. Oddly enough, Sinister trusts himself more than any other mutant, so he will most likely help his doppelgänger in his plan.
Combined with the information coming from X-Men: Hellfire Gala #1, there are four Mr. Sinisters walking around Earth. Each one maintains they are the original, but there's no realistic way to tell. Unlike Spider-Man's Clone Saga, Mr. Sinister is excited to meet his clone selves - because he's just as interested in solving the mystery as the reader, and he sees the entire X-Men affair as a simple game.