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Luigi Mangione is keeping himself going behind bars thanks to some surprising ladies -- Taylor Swift and Charli XCX .

In a new fan letter circulating online and verified by TMZ, Luigi explained he recently downloaded their music to his prison tablet at Brooklyn's Metropolitan Detention Center -- even admitting he’d never really listened to them before, but decided to give it a shot after a fake list of his "favorite artists" went viral. Getty

Turns out, the alleged murderer of UnitedHealthcare CEO Brian Thompson actually tried out Taylor’s "cardigan" while walking laps on the tier... until another inmate, known as "King," caught wind of it.

According to Luigi, "King" wasn’t having it, roasted him for his song choices and swapped his playlist -- and now he

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