In 2001, the death of his father sent him into a downward spiral. Duke struggled with alcohol and depression. By 22, he was homeless.
"I always remember just feeling so alone," he said.
At a shelter, he saw a recruiting poster for the Homeless World Cup, a soccer tournament for homeless people. Duke made the team, and while the 2004 tournament was a thrill, he realized that the training for it had been the real catalyst for changing his life.
"The structure of training, the camaraderie with my teammates, having positive role models -- it all got me back into a positive place," said Duke, now 36.
In 2009, he started Street Soccer Scotland to share what he'd learned with others.
Every week, his organization provides free soccer clinics to more than 1,000 people from disadvantaged backgrounds. Participants may be struggling with homelessness or addiction; some are refugees experiencing social isolation.
"When you're homeless or on the edge of society, often you don't feel part of anything," Duke said. "But football gives you a place where you belong ... and that's when change happens."
Duke also partners with organizations that offer assistance with housing, addiction and mental health. But at its heart, Street Soccer is about giving people a chance to connect with others and move themselves beyond difficult times.
"We tell them 'It's not about what's happened yesterday. It's about what you do tomorrow' " Duke said. "That's the best inspiration for anyone."
CNN spoke with Duke about his work. Below is an edited version of the conversation.
CNN: The Homeless World Cup played a pivotal role in your life. What was that like?
Duke: At that time, it was only in its second year. A street paper for homeless people was assembling a football squad to represent Scotland. It took quite a lot of courage for me to go to the tryouts because I had no self-esteem and was feeling so low.
For three months, we trained twice a week. It gave me structure, friendships, confidence. It gave me something to look forward to -- a wee bit of hope. It got me my life back.
After the tournament, I started coaching kids' football and decided to go to college. I trained as a community support worker and got a job working with people who were on the streets. I was the assistant coach for the Scotland Homeless World Cup team for a couple of years and was the head coach in 2007, the first time Scotland won the tournament.
But I realized, while the (event) is great for the players who go to represent their country, in Scotland, there are 36,000 homeless people. So I quit my job and set up Street Soccer to create something that's year-round. And we use the Homeless World Cup as this kind of celebration once a year.
Just last month, we held the 14th Homeless World Cup here in Glasgow. It's the most inspiring football tournament in the world.
CNN: Your program is about more than playing football.
Duke: We see ourselves as football for everyone -- a place where you belong. Once players build up confidence at our weekly drop-in football sessions, they often say, "What else can I do?" So we run education courses in communications, writing skills or working with others.
Players who have been engaged for a few months can volunteer with our youth program in areas affected by poverty. All pass background checks and receive additional training. It's a unique solution because, on one hand you've got kids whose parents maybe can't afford to send them to a local football club, and on the other, our players are getting a chance to give back and gain skills. So it's a win-win.
CNN: How successful is your program?
Duke: Over 80% make some sort of positive impact in their life, whether that's getting a job, maintaining a house, addressing addiction or improving mental health.
But Street Soccer's not a magic wand. You just don't wave it and people suddenly change their lives. There's a lot of effort and a lot of hard work that goes in from the players and a lot of support from the staff, coaches and volunteers. But if we all help one person each time, then that's a lot of people.
CNN: Your personal story is powerful evidence that football can change lives. How has that carried over to your work?
Duke: When I started, I was the homeless person who'd changed his life. But now, most of our staff has come through the program -- through addiction or homelessness. So players can see someone who has walked in their shoes. They can see a way out. It emphasizes the idea that, 'Maybe I can change.' And that's all we're about.
Want to get involved? Check out the Street Soccer Scotland website
and see how to help.