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Playboi Carti got into some trouble while on tour -- he was charged with misdemeanor assault during his time in Utah earlier this month, law enforcement says.

Here's what we know -- cops say a limo driver named Carl Reynolds told them the "Rather Lie" rapper assaulted both him and the musician's girlfriend October 2 while he was driving him and his entourage from the Waldorf Astoria in Park City to rehearsals. Carti was in town for his Antagonist Tour at the time. Gallery Launch Gallery Getty

Carl claims things took a disastrous turn when Carti and his other half got into a heated argument -- apparently over sex -- in the back seat ... and he says Carti threatened to beat up both her and Carl after he intervened.

TMZ is told Carl says Carti wound up punching his GF in

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