Digital Spy reports that his fiancé Omarosa Manigault-Stallworth was visiting him, but had just stepped out for a period of time and returned to find that the actor had passed away.
He had suffered a heart attack in July.
The actor's representative said: "[Omarosa] is grateful for all of your prayers and asks for privacy at this time. Celebrations of his life, both private and public, will be announced at a later date."
Duncan was nominated for a Best Supporting Actor Oscar for his role as John Coffey in hit prison drama The Green Mile - and also appeared in Armageddon, The Whole Nine Yards, Planet of the Apes, Sin City and Daredevil.
Before he got his break into acting he had worked as a bodyguard for celebs such as Will Smith, Jamie Foxx, LL Cool J and Notorious B.I.G.