Characters meeting alternate versions of themselves is a common trope in Marvel comics.
There are too many instances of this to list, but here are a few: The Hulk most famously fought a future version of himself ( Maestro ) in the The Incredible Hulk: Future Imperfect series.
The Maestro character would later appear several other times as well, always posing a dangerous threat to the younger Hulk.
If you count the psychic plane, the various incarnations of The Hulk had to face a new incarnation - Guilt Hulk -
within Bruce Banner's mind in Incredible Hulk #377 .
The Guilt Hulk, an evil persona based on Bruce's abusive father, would show up again later as well.
The original five X-Men were brought forward into the future in the All-New X-Men series, resulting in all but Jean Grey meeting themselves.
This proved particularly traumatic for Scott Summers , who ended up fighting his older self on several occasions.
An offshoot of the Exiles, the Wolverine Squad is a team made up entirely of Wolverines from different universes.
The team was created specifically to kill a gestalt being - Brother Mutant - which included yet ANOTHER version of Wolverine in its makeup.
The current " Secret Wars " arc witnessed multiple characters meeting alternate versions of themselves
In the new reality ruled over by Doom, his will is enforced by The Thor Corps - a police force comprised entirely of Thors from different universes.
The X-Termination event saw X-Men from three different continuities interact.
These are just a few - there are numerous examples available.
And if you count cases where the doppelganger is an android, clone, shape-shifter, or something else assuming the form of a character, the list becomes even larger.
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The Ultimate Fantastic Four had the unfortunate luck to meet their Zombie-Universe counterparts in Ultimate Fantastic Four Vol. 1, issue 21.
This was the beginning of the Marvel Zombies storylines.
I'm certain there are other instances of Marvel characters meeting other-universe counterparts, but this one sticks out in my mind.
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Here are three panels from Alan Moore's run on Captain Britain in which the hero meets numerous alternative versions of himself.
There was later an entire superhero team formed of alternative versions of Captain Britain, the Captain Britain Corps .
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Not if you include Beta Ray Bill/
Beta Ray Thor as an alternate version of Thor:
Thor has also met this alternate version of himself:
And he once turned into a Hulked-Out version of himself:
And as cde pointed out, in Thor Corps, Thor teamed up with Beta Ray Bill, Thunderstrike, and Thor 2099.
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Xmen travel through alternative time lines like squirrels cross branches of a tree.
Cable has met Nate Grey the X Man, and Strife.
Beast was kidnapped and tortured by Dark Beast.
Gambit fought Sun, a physical god version of himself.
Storm and Wolverine have met their days of future past punk counterparts.
Wolverine has met I think, his gay and in love with Hercules alternative self.
SheHulk has met and interrogated her "Slept with Juggernaut" copy.
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The X-man met the X-babies, alternate versions of themselves created by Mojo.
Jean Grey and Madeline Pryor. Magneto and Joseph
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As I understand it, Spider-Man has met alternative versions of himself on several different instances, such as the Noir, 2099, & Ultimate alternates, at one point teaming up with them and other alternates as Web- Warriors. Deadpool also has encountered several alternate versions of himself, going so far as to create an entire team of alternate Deadpools known as the Deadpool Corps. I was just curious: have any of the other Marvel characters experienced similar encounters, or has it only been Spidey and the lovable Merc with the Mouth?
Are Spider-Man & Deadpool the only characters who have met alternate versions of themselves?
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Characters meeting alternate versions of themselves is a common trope in Marvel comics. These include the Hulk, Thir, most of the X-Men,the Fantastic Four, Beast, Storm, and Wolverine. With the Hulk at the helm, the list of Marvel characters who meet alternate versions of themselves also includes Thor, Cable, Beast, Gambit, Storm and Wolverine, SheHulk, the X-Men, Jean Grey and Magneto. Moreover, Marvel characters are known to meet alternate versions of themselves in All-New X-Men, Secret Wars, X-Termination and the Ultimate Fantastic Four. 
It is commonplace for Marvel characters to meet alternate versions of themselves, the Hulk being the most famous. Marvel characters are known to meet alternate versions of themselves in All-New X-Men, Secret Wars, X-Termination and the Ultimate Fantastic Four. Marvel characters who meet alternate versions of themselves include the Hulk, Thor, Cable, Beast, Gambit, Storm and Wolverine, SheHulk, the X-Men, Jean Grey and Magneto. 