“He did the legal research,” Mr. Rideau said. “He put together the case. I would not be here but for Calvin. But I’m not the only guy. He got other guys out of prison, too.”

Mr. Duncan was also a resource for real lawyers. “I had a legal question I couldn’t figure out,” said Katherine Mattes, now a law professor at Tulane University, recalling her early days representing prisoners challenging their convictions. “I started asking capital defense lawyers around town. They said: ‘I don’t know the answer to that. Go ask Calvin Duncan.’”

She went to see him at the prison. “I ask him the question,” she said. “He immediately, off the top of his head, gives me the case name and the citation that answers the question.”

While he was in Angola, Mr. Duncan was once allowed to visit Tulane’s law library. “Before prison I had never set foot on that campus,” he said. “I said back then, and that was a long time ago, that I wanted to go to that university.”

He was released on a Friday. “That Tuesday, I was on the Tulane campus, trying to figure out how to go to school there,” he said. “I was too late for the spring, but I enrolled for the fall.”

He graduated last year. “I was told I needed my bachelor’s degree to go to law school,” he said. “That’s what I’ve been working on.”

On the outside, Mr. Duncan continued to question the Louisiana law allowing non-unanimous verdicts. “I went to the law library,” he said. “I discovered that the law had been introduced for the purpose of making sure that white supremacy stayed alive.”