Michael Clarke Duncan Could Have Saved Biggie

Notorious B.I.G. could still be alive today ... if only Michael Clarke Duncan hadn't switched shifts the night he was killed.

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"The Green Mile" star who once did security for rappers, told us he was scheduled to work for Biggie Smalls on March 9, 1997 -- the night he was killed in a driveby -- but switched gigs with a friend to guard Babyface.