Turns out the mutant in question, either A. Faked his death or death was faked. B. Survived or put in a state of sleep that resemble death, or the various other reason you can think in a comic book universe. and the 5 revived that mutant, is it possible? I could imagine how awkward it would be to have 2 of the mutant running around. Im sure if anyone sensing the death in the first place bound to make a mistake.