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Tupac Shakur had a promiscuous reputation in the early 1990s ... and a much-anticipated new book claims it drove Janet Jackson to ask the studio to have him tested for AIDS before a kissing scene.

Jeff Pearlman -- a noted biographer -- has written a new book titled "Only God Can Judge Me: The Many Lives of Tupac Shakur" ... and, in it, he touches on Tupac's time filming the 1993 flick "Poetic Justice" with Janet.

According to Pearlman, the two didn't get along much ... with Janet a consummate professional on-set while Tupac had a tendency to come and go as he pleased. Everett Collection

Days before a kissing scene between Pac and Janet, Pearlman writes, Tupac apparently kept talking about "overwhelming Jackson with his lips and mouth" ... and, ultimately, S

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