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.

"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.