The Oscar-winning actress, who died in late March at 69, was honored at a memorial service in Coeur d'Alene, Idaho, where she had built a peaceful life over the last 30 years alongside husband Mike Pearce and their four dogs (and where she was known, simply, as Anna, her birth name).
The public service, at Lake City Church, was attended by friends and family alike.
Melissa Gilbert, who starred alongside Duke in 1979's The Miracle Worker, said, "From the moment we met, we were thick as thieves and we remained that way for 40 years."
"I hate that I'm here, and yet I love that I'm here to celebrate my dear teacher friend, Anna," Gilbert said.
Duke's son Mackenzie Astin honored her advocacy for mental health, following her own biopolar disorder diagnosis, and said, "She put a beautiful face to the illness."
Gilbert echoed that, in part: "Anna loved to laugh," she said.
"She had courage like no one I've ever known."