The hint of swagger in his walk might reflect that past or his eminent present.
"He is the concertmaster of China's labor movement," said Beijing-based scholar Wang Jiangsong.
Duan has helped workers at one of the world's busiest ports negotiate for better pay and benefits, shown laborers at an Apple supplier in southern China how to establish a union branch, counseled Wal-Mart employees battling for payouts, and advised striking workers at IBM and Nokia how to protect their rights during ownership changes.
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Workers flock to his modest 26th-floor offices in Shenzhen, southern China, to hear advice sometimes peppered with profanity.
That advice is that Chinese workers have the right to organize and, if necessary, strike. And more and more seem to be listening.
China Labor Bulletin, a Hong Kong-based watchdog with whom Duan cooperates, recorded 1,171 strikes in China from June 2011 through 2013. This year it has tracked 1,213 so far. That includes China's biggest strike in decades, involving 40,000 workers at a company that supplies Nike, Adidas and other global brands in April.
After setting up Laowei Law Firm in 2005, Duan and his colleagues have helped hundreds of individuals fight employers, but he soon realized that the problems they faced - unpaid arrears, lack of job security, inadequate social insurance payments - could only be resolved by collective bargaining, backed by the threat of action such as strikes.
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Duan believes he has on occasion sailed close to the wind.
Late last year, he said he received warnings that the police had developed "opinions" about him in connection with a case he was working on, and that he ran the risk of detention.
Through a friend in the capital - a member of the "Second-Generation Reds", the children of China's Communist revolutionaries – he passed a letter to President Xi explaining his work. Thereafter, he said, the pressure eased.
He sometimes draws criticism for endangering his clients, too, such as security guards he was advising at a Guangzhou hospital, who were arrested after staging a rooftop protest.
Ma Jianjun, a lawyer who considers Duan a friend, says he has great respect for this "man of ideals" but doesn't give him a free pass.
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"To a very large extent he is half-lawyer, half-social activist. From a lawyer's perspective he is unprofessional," he said.
"He's a controversial but impressive figure," said Mary Gallagher, associate professor of political science and China labor expert at the University of Michigan.
"I think people are really interested to see how far things go."