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The Jonas Brothers left Disney fans stunned at their New Jersey concert ... bringing out a surprise guest that had everyone feeling nostalgic.

It all went down Sunday night at MetLife Stadium ... where the JoBros were reflecting over their 2008 smash hit Disney movie, "Camp Rock." Joe Jonas -- the film's lead -- broke out into song, singing "Gotta Find You," sending fans into a frenzy.

But then ... he brought out Demi Lovato on stage -- who played his onscreen love interest -- and the whole crowd went wild.

The duo then belted out some of the most popular songs from the movie like "Wouldn't Change a Thing" and "This is Me."

It was a long time coming for this reunion ... after "This is Me," Demi and Joe told the crowd they haven't performed the song live in roughly a decade.

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