Jeffery Goines (12 Monkeys) and Tyler Durden (Fight Club) are the same person.

https://www.reddit.com/r/FanTheories/comments/12jczl/jeffery_goines_12_monkeys_and_tyler_durden_fight/

Watching 12 Monkeys, I began to see some eerie similarities between Brad
Pitt’s performance in that and his performance in Fight Club. The closer I
looked, I realized that they just might be the same person. Here is what I
believe took place between when James Coles first encounters him in 1990
and when he caught up with him again in 1996.

In 1990 Jeffery Goines is institutionalized in Baltimore where he meets
James Cole and has the spark of inspiration that would eventually become
the Army of the Twelve Monkeys. With his penchant for troublemaking and his
involvement in Cole’s subsequent escape, Jeffery is severely punished. The
punishment comes in the form of clinically administered electroshock
therapy. The barrage of treatment ultimately induces a psychotic break and
the splintering of his personalities, one being a more mild mannered and
one more rebellious, I.E. “Jack” (or Joe from the novel) and “Tyler
Durden”. As “Jack” he convinces the doctors that he’s now sane.

Upon release (let’s say circa 1992) he moves away from Baltimore,
presumably to the west coast, to start his life anew. He gets an office
job, a condo, and tries to live what he presumes is a normal life. Away
from the hospital, his “Tyler” alter becomes stronger. “Jack” begins having
sleep problems and, upon his doctor’s recommendation, starts to attend
support groups. The therapeutic setting affects “Jack” subconsciously,
reminding him of the institution, and represses “Tyler’s” growing presence.
When Marla starts to crash his groups, “Tyler” finds another agent of
social sedition and again grows stronger.

Not long after “Tyler” makes his presence known to “Jack” and starts to
deconstruct his buttoned-down life. “Jacks” condo explodes, he moves in
with “Tyler”, and Fight Club begins. Eventually Tyler begins Project
Mayhem. The members of Project Mayhem, dubbed “Space Monkeys”, begin a
series of terrorist style mischief making including seeding a BMW lot
causing pigeons to shit on all over the lot, painting an enormous smiley
face on the side of a building, and eventually abducting and threatening an
elected official when the police threaten to shut down Fight Club.

After “Tyler” enacts a plan to blowing up the headquarters of the major
credit agencies, “Jack” decides to take matters into his own hands, and
shoots himself in the face.

This is one of the places where the book and the film diverge. In the book
“Jack” (or presumably Jeffery, since the trauma of the shooting has mended
his fragmented personalities) wakes up in the hospital where he encounters
members of Project Mayhem. They get him transferred back to the Baltimore
institution (most likely around 1994). Since the destruction of the credit
buildings has sparked a federal investigation, lesser members of Project
Mayhem take the fall clearing Jeffery of any suspicion.

He is soon released back into his father’s care and quickly reconnects with
his top “Space Monkeys” disciples from Project Mayhem and creates the Army
of the Twelve Monkeys. They again start a mischievous war on the
establishment, which ultimately culminates in the abduction of Jeffery’s
father, prominent biologist Dr. Goines, and the releasing of all the
animals in the Baltimore Zoo. Unfortunately, this would be their last big
“prank” as all members of the Army of the Twelve Monkeys, as well as most
of the human race, becomes mortally infected by a deadly super virus.

TL:DR Between 1990 and 1996, Jeffery Goines’ personality splits into “Jack”
and “Tyler Durden.”
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