The certificate, issued September 10, lists respiratory failure as the immediate cause of death, preceded by "anoxic encephalopathy" (not enough oxygen getting to the brain), "ventricular fibrillation" (an abnormal heart rhythm and "acute myocardial infarction" (a heart attack).
The document also states that The Green Mile star had sarcoidosis, a disease that can cause inflammation in the lungs, eyes, skin and other tissues.
Last week, the L.A. County coroner's office announced that they did not see any reason to review Duncan's death, as all evidence suggests that he died of natural causes.
The Oscar nominee died at Cedars-Sinai Medical Center on September 3 after being hospitalized for a heart attack suffered on July 13. He was 54.