The 88-year-old author and grandfather is surrounded by papers and notes he has made about his remarkable ancestor and as he talks, the story of Robert Campbell comes to life.
"He was a fascinating person," explains Duncan, who lives in Ardrossan.
"I am full of admiration for him. And yet not much is known about him in Scotland, the land of his birth."
He adds: "When I was a boy I didn't realise he existed.
"It was only when I started to research my family tree, for the sake of my own children and grandchildren, that I discovered his wonderful story."
Robert Campbell was born in Perthshire on February 21, 1808, the son of a sheep farmer.
In later life, his children would attend school in Glasgow, and he often spent family holidays in the city.
Entranced by tales of "the great north-west" from his cousin, Robert joined the Hudson's Bay Company, firstly as a farmer and then as a fur trader.
In his memoirs, he writes: "The boundless prairies roamed by tribes of Indians and herds of buffalo, the vast lakes and giant streams, the sublime majesty of the Rocky Mountains....all was a revelation to me."
The reality was a little different, however, as Robert discovered the harsh conditions of living in the region.
In constant danger of attack, he and his men also faced starvation during tough winters.
He describes having to eat the "lacing of our snow shoes" to stay alive and on one occasion, talks about the terror of learning his camp had been under threat.
"The hostile tribe....confessed that had I knelt down to drink they would have rushed upon me and drowned me in the swift current and ...would have massacred the sleeping inmates of my tent..." he writes.
Duncan explains: "Robert had some extraordinary adventures. He filled in the gaps on the map of North America - the rivers and prairies he travelled had not been documented before.
"And he never named any of them after himself, always after friends or people he held in high regard."
Robert and his wife Eleonora, who died from cholera in 1871, had three children.
After his retirement he lived with one of his sons in Manitoba and was the first to import Highland cattle into the province.
He died on May 9, 1894.
It was not until after his death that Mount Campbell was named by the Gold Commissioner of the Yukon, and even later - the 1960s - when the 362-mile Robert Campbell Highway was named in his honour.
The only known plaque in Scotland is on a cairn near Robert's family farm, erected by locals with help from Duncan.
The former Millport town clerk is now working on a book about Robert in a bid to raise awareness of his ancestor's exploits.
Duncan is already a respected author and historian, having published Millport and the Cumbraes - A History and Guide, which many consider to be the definitive local history guide to the area.
His books, which he writes as J R D Campbell, also include Some Cumbrae Yesterdays, a richly illustrated look at the history of the island and Clyde Coast smuggling.
"I didn't start writing until I moved to Millport," he explains.
"I was a travelling financial inspector for the Hydro Board, which took me all over the country, so I was too busy.
"But when I learned there were plans to get rid of lots of local history records from Millport, I decided to create my own historical record, and it grew from there."
He smiles: "And to think when I retired, my wife wanted to keep me busy...."
Duncan's wife Betty sadly died recently, and he moved to Ardrossan where he continues his research and writing.
His son Angus and daughters Judith and Christine, and their families, were astonished to learn of their famous ancestor.
"I'd like the book to help them understand the story of Robert Campbell," says Duncan.
"And if it went to a wider audience, then that would be fantastic.
"His travels did have a huge impact on the area. He improved trade facilities and even administered medical advice, assisted by his wife.
"In one day she administered more than 100 smallpox vaccinations, for example. And he was considered very strong, with great stamina.
"Apparently, he swam every day in the Yukon, even if he had to break the ice to do so. And the first headache he had was the day before he died..."
Duncan admits researching his family tree - which has taken more than 20 years - was a labour of love.
He laughs: "I'm quite nosey - as my family will tell you - and I like to get to the bottom of things. But I never expected to find Robert. He is something special."