Jamie Madrox, a.k.a. Multiple Man started out as a member of Moira Mactaggart's pet mutants on Muir Island, along with Siren, and her and Xavier's son, Legion. Madrox later became a member of X-Factor, a group founded by former X-Men members, pretending to be government sponsored mutant hunters. By the time Jamie joined the team, this charade had been ended, and X-Factor was just another X-team.
Jamie died several times, as noted above, and then finally perished for good from the Legacy Virus, which first caused his duplicates to gain personalities and refuse to reintegrate with him, and eventually caused his death.
And, then, surprisingly enough, he came back to life, and currently is featured in Marvel Comics' Marvel Knights line of comics.
Jamie's power is to convert kinetic energy directed against him (a punch was sufficient) into a duplicate or duplicates of himself. He had no control over his power, which forced him to wear a specially designed suit to muffle impacts, and allow him to move and act normally without leaving a trail of duplicates. The suit had a window in the palm, and was open at the face, so that he could create duplicates when he wanted. These duplicates were, generally, subservient to Madrox, but have on several occasions developed personalities and drives of their own, even in situations not involving the Legacy Virus.