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It's the summer Jenny Han duped fans into thinking they knew how the finale ended ... and she's pretty tickled by her fake-out.

"The Summer I Turned Pretty" creator confirmed she and the cast DID film fake scenes while in production in Paris for the third and final season ... admitting it was a way to protect the story from being spoiled.

ICYMI ... 'TSITP' fans were sent into a tizzy last year when Lola Tung -- who plays leading lady Belly Conklin -- was seen reuniting with Gavin Casalegno -- who plays Jeremiah Fisher -- for one scene in the City of Light ... indicating the two were endgame.

But, as fans of the show will know, the scene never came to fruition and Belly ended up with -- SPOILER -- fan-favorite character, Conrad Fisher, played by Christopher Briney.

So, af

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