EXCLUSIVE

The Duffer Brothers have a smoking gun to prove they didn't rip off "Stranger Things" ... TMZ has learned.

The brothers are being sued by Charlie Kessler, who produced a short film back in 2012 called "Montauk." He claims he had a meeting with the Duffers in 2014 where he pitched them a series based on the short film. He says he never heard back, but was shocked when "Stranger Things" debuted in 2016, using his script idea and story.

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TMZ has obtained emails showing the Duffer Brothers were developing a "Montauk" series as far back as November, 2010 -- 3 1/2 years before Kessler claims he even pitched the show to them.

In 2 emails in November, 2010, the brothers set out their vision ... a "real," "paranormal" "gritty eighties" show. In one of the emails, it specifically says the show is set in Montauk. There is a reference to "the Montauk experiments."

A Google document dated October, 4, 2013, would become the premise for "Stranger Things" ... "Benny (renamed Will for the show) leaves his friend Elliot's house, a bunch of kids are there, eating pizza, dungeons and dragons ... Benny leaves on bike, hears voices, goes into strange world, taken by some evil force."

Two emails from 2014 -- February 27 and April 4 -- both written before the alleged meeting between the Duffer brothers and Kessler, lay out the vision for the show. It reads, "...set in 1980s Long Island" with "a vintage Stephen King feel." The April 4 email mentions a location scout in Montauk.