RIP. Big people like him have shorter lifespans, it was expected. Although he lived 9 more years than Andre the Giant.
They don't really have that much of a reduced lifespan.
If they are like Andre the Giant, as in, have a definitive case of acromegaly and go untreated, they definitely don't stand much of a chance to live into the elderly years. But they can still live to the "average expected age" and beyond.
Richard Kiel ("Jaws" from Moonraker, etc) is 73 or thereabout.
Oddly enough, according to the wiki page on acromegaly, Tony Robbins has the disease (or had it, and got it treated early). He's tallish, Duncan's height, but thinner framed. Probably had it treated very early, or otherwise a mild case.
But I'm not seeing anything anywhere, including Google, suggesting Duncan had either gigantism or acromegaly.
I think he was just a big guy - and plenty of big guys live long.
And many people die young. Maybe he had guilty pleasures regarding certain foods or consumables? Maybe he just had the bad luck card drawn and it was just the way things worked out?