Warning: This article contains spoilers for X-Corp #3!
Out of all the X-Men in the Marvel Universe, Jamie Madrox - Multiple Man - has one of the most unique and powerful mutant powers. With the ability to duplicate himself dozens and potentially even hundreds of times, Madrox can become a literal one-man army... but this army suffers from a crippling weakness. The savior of the island of Krakoa can also bring about its end.
Multiple Man is unlike any other mutant in existence, beginning with his powers manifesting at the moment of his birth (most mutant powers manifest when the mutant is a teenager). Every impact on Jamie's body creates an exact duplicate of himself, but he can also create them at will by clapping his hands, snapping his fingers, etc. The duplicates have free will but share a psychic link with the original Jamie, who, upon reabsorbing the duplicates, obtains all memories and skills learned by each. Jamie initially rejected Professor X's invitation to join the X-Men, but recently he's become an indispensable part of the mutant island of Krakoa. Unfortunately, Multiple Man - all of him - can be taken down by the most mundane of weaknesses: a simple allergy.
A primer of sorts appears before X-Corp #3 - written by Tini Howard with art by Valentine De Landro and colors by Sunny Gho - which describe Jamie's powers in great detail. The entire X-Corp lab is staffed only by him - or rather, his duplicates, who must follow precise instructions left by the original. Along with a complicated weeklong schedule, Jamie's notes shed light on exactly how his powers work - and how they don't. In particular, a small footnote at the bottom of his instructions reads "Believe it or not, some of us do actually have different food allergies and tastes, so please be respectful in the kitchen." It's a small note in a rather large instruction letter, but the implications are quite deadly.
This is proof that not all of Jamie's duplicates are biologically identical. Readers already know that Jamie has occasionally sent his duplicates out into the world for months or even years at a time, each learning different skills (one of whom even became a priest that Jamie left alone, refusing to reabsorb him). If one duplicate develops an allergic reaction to an unknown allergy, and Jamie reabsorbs that duplicate, it's possible that Jamie can be compromised, along with every subsequent duplicate. In current comics continuity, many copies of Multiple Man are needed to run and maintain the mutant haven Krakoa and without him, the island would simply lack the manpower to function properly.
Odd weaknesses are par for the course when it comes to comics, but Jamie's weakness is also a unique insight into how his powers function when the proverbial many become one. A weakness in one member could hypothetically bring down the whole team, and even the mutant's new Krakoa nation with it. For the sake of all mutants everywhere, it's a good thing that Multiple Man instructs his duplicates well concerning kitchen etiquette in the workplace.