And now five decades after the series began, the Oscar winner ("The Miracle Worker") is coming clean: "I liked Cathy better," she confessed by phone interview from Honolulu, where she's appearing in the play "Heaven Forbid!"
"Patty was very embarrassing to me," the 66-year-old said, laughing. "They would write those outrageous things for her to do. I would dive in and do it, but I couldn't wait to get to Cathy. She was sensible, and I'd like to think she was classy."
The show had a slightly surreal concept: look-alike cousins? It had a catchy theme song -- "Where Cathy adores a minuet / the Ballet Russes and crepes suzette / our Patty loves to rock and roll / a hot dog makes her lose control -- what a wild duet!"
It also boasted such guest stars Sammy Davis Jr., Peter Lawford, Frankie Avalon, Sal Mineo and the British pop duo Chad and Jeremy.
"I was obsessed with them," said Duke of Chad and Jeremy. "That was a big week for me."
The series is so much a part of the pop lexicon, the show was even seen beaming on the TV screen on a recent episode of "Mad Men."
Though she played the carefree Patty onscreen, offscreen her life was a nightmare because of the viselike grip her Svengali-esque managers John and Ethel Ross had over her.
"They were very misguided people," said Duke, who has been a mental health advocate since she was diagnosed with bipolar disorder in 1982.
She wasn't even allowed to watch the series when it was on. So it wasn't until the 1980s that she finally saw it.
"One day I was clicking around the channels and all of a sudden there is Patty. The first thing I thought was what is that ugly hair? They actually let me go out like that?
"But by the end of the show, I realized it did have some value. Patty was doing naughty things, but she had respect for her parents."
She has remained close to William Schallert, who played Patty's loving and often exasperated father, Martin Lane. "He's the dad I never got to spend time with," she said.
She said she's tickled when baby boomers approach her about the series. "There is a community or a coming together that I treasure. When someone comes up to me -- and they might have white hair like mine -- and they say, 'I grew up with you,' I say, 'Now we're growing old together.'"
Duke's also treasuring that four of the albums she recorded in the 1960s have made their CD debuts. The folk song album, which Duke recorded in 1968, was never released.
Arnold Goland, music arranger and conductor on her first albums, said Duke could definitely carry a tune.
"I don't think Judy Garland or Barbra Streisand were worried about the competition," said Matt Tunia, who was the CD reissue producer, "but the charm she brought to those records is captivating and real fun."
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