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Offset's looking a lot like UFC champ Conor McGregor -- and not in a good way -- after slapping the crap out of a fan's phone ... and the fan says he's getting the police involved.

18-year-old Junior Gibbons says he was at a Target in Sandy Springs, GA on Monday when he saw the rapper buying a baby stroller. Gibbons took out his phone to record a selfie video, but when he asked, "What up, Offset?" ... he didn't get the response he wanted.

Instead, he says he ended up with a cracked phone.

After his phone slammed to the ground, Junior claims Offset told him to get that "f***in' phone out of my face" ... and then walked out of the store.

Junior tells us he went to the cops and filed a police report Friday, but says his family just wants Offset to pay to replace the phone.

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You'll recall, McGregor had a very similar incident last month outside Liv Nightclub in Miami. He was eventually charged with robbery and criminal mischief, and his trial's set to begin next month.

We've reached out to Offset ... no word back, so far.