The Green Mile actor, who was a security-man-for-hire before his acting career took off, has confessed he was assigned to guard the heavyweight star at the Soul Train Music Awards in Los Angeles in March, 1997, but he swapped the job with a pal to cover R&B singer Kenneth 'Babyface' Edmonds instead.
Notorious B.I.G., real name Christopher Wallace, was gunned down in a drive-by shooting hours later.
And Duncan admits the incident was a rude awakening - one which prompted him to make a career change.
He explains, "One of my last assignments, it was really bad, because we were doing the Soul Train Music Awards and I was assigned to Biggie Smalls that night. The night that he got killed. A guy called me at the last second and said, 'Did you want to switch?' So I switched and went to Babyface and my friend went to Biggie Smalls - and that was the last time I worked security."
And the actor can't help but think that the rapper would have survived the attack on his life if he had been in charge of his security for the night.
He says, "In hindsight, everybody says I would have done this (differently) but my thing was never let the client sit on the passenger side door in the front. If you were my client, I'd have you... sitting in the middle (seat).
"When things like that happen you have to ask yourself, 'Am I really ready to give up my life to do this?' And I figured if you're not ready to do that, you need to find some other type of work."
Notorious B.I.G.'s murder remains unsolved.