She won an Oscar at 16 for playing Helen Keller in

The Miracle Worker

, but for fans of a certain age - and those who caught her 1963-66 show decades later on Nick at Nite - Patty Duke will always be Patty

and

Cathy Lane.

"One pair of matching bookends, different as night and day," they were the "identical cousins" of The Patty Duke Show, their differences reportedly inspired by cocreator Sidney Sheldon's having noticed different sides to the young star's personality.

Decades later, she was diagnosed as bipolar.

Ms. Duke, an actress and writer who inspired many with her openness about her mental illness, died early Tuesday of sepsis from a ruptured intestine. She was 69.

Born Anna Marie Duke on Dec. 14, 1946, she had an unhappy childhood, was abused by her parents, and, by age 8, was largely under the control of managers who supplied the young actress with alcohol and prescription drugs.

Through the then-magic of split-screen television, though, she lived not one, but two, idealized adolescences.

As Patty, who, as the earworm of a theme song went, had "only seen the sights a girl could see from Brooklyn Heights," Ms. Duke was the all-American daughter of a newspaper editor (William Schallert) and his wife (Jean Byron). As Cathy, who'd "lived most everywhere, from Zanzibar to Berkeley Square," she acquired a globe-trotting father (also William Schallert) and a slight Scottish accent.

After the show ended, Ms. Duke left the Lane girls behind with a starring role in the 1967 adaptation of Jacqueline Susann's soapy Valley of the Dolls, but television movies, series, and guest appearances remained a mainstay of her career.

Notable roles included Martha Washington in the 1984 mini-series George Washington, some of which was filmed in Philadelphia, and in its 1986 TV movie sequel; as the first female president in the short-lived comedy Hail to the Chief; and as a minister in another short-lived series, Amazing Grace.

In 1990, she played herself in Call Me Anna, a TV movie based on her 1988 memoir, and one of her three Emmys was for a 1979 TV remake of The Miracle Worker, in which she played Annie Sullivan and Melissa Gilbert played Keller.

Offscreen, Ms. Duke's life was at times turbulent. She married four times (one brief marriage was annulled and all but the last took place before her 1982 diagnosis). She was the mother of three sons, actors Sean and Mackenzie Astin, and Kevin Pearce.

She became an advocate for the mentally ill and in recent decades appeared to have found happiness in Idaho, where she lived with husband Michael Pearce.

This month, Ms. Duke had gone on Twitter to note their 30th wedding anniversary, reporting, "We are having a very happy and quiet day."

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The article contains information from the Associated Press.