That is surprising; I would imagine that studios would write into an actor's contract that they would be unable to play another similar character in a different movie. (insert movie trivia here) I remember that Roger Moore was blocked from playing in any additional 007 films back in the 80's because he played as James Bond (in appearance if not by name) in a spoof called "Cannonball Run". Ever since that time, any actor playing James Bond has been required to never play a similar character in any other movie in which they appear.
For the Green Lantern/Deadpool situation, this means one or more of the following:
Green Lantern and Deadpool aren't too similar to warrant such a contract addendum; or
They're not expecting sequels for any movie in either franchise; or
The actor playing the characters is replaceable/dispensable, and won't disrupt character continuity much.
Well, I never thought of it that way,
Deadpool has a fill face mask I think,
and the Lantern is kinda like the Phantom, it gets passed from person to person, like a lot of super hero types,
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Time for a Sexy Party!
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Something even more odd happened with Fantastic Four and Captain America that are both Marvel movies.
Chris Evans plays The Human Torch in the Fantastic Four movie and the title part in Captain America. So he plays 2 different superheroes in 2 different marvel movies, in the same Marvel universe I'd guess.