Duke's agent, Mitchell Stubbs, says the actress died early Tuesday morning of sepsis from a ruptured intestine. She died in Coeur D'Alene, Idaho, according to Teri Weigel, the publicist for her son, actor Sean Astin.
Duke, born Anna Marie Pearce, followed on her early success playing the young Helen Keller with a popular sitcom, "The Patty Duke Show," which aired for three seasons in the mid-1960s. She played dual roles under an unconventional premise: identical cousins living in Brooklyn Heights, New York.
In 2015, she played twin roles again: as a pair of grandmas on an episode of "Liv and Maddie," a series on the Disney Channel.
IN MEMORIAM: Celebrities and notable figures who have recently passed away