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Rapper Fredo Santana died Friday night from a seizure ... family members tell TMZ.

The Chicago rapper who is a cousin of Chief Keef was at his L.A. home Friday. We're told his girlfriend came over at around 11:30 PM and found him dead on the floor.

Santana had been hospitalized recently for liver and kidney problems he'd been battling for months.

The rapper had talked openly about his lean addiction ... as we've reported way too many times, lean can trigger seizures.

A number of rappers including Drake have gone on social media the last few hours to pay their respects.

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He had an 8-month old son.