He is the son of late Raymond Twomey Duncan, an oil and wine entrepreneur who co-founded Silver Oak in 1972 with Justin Meyer.

Silver Oak is a family-run and owned business, and his brother Tim Duncan is Executive Vice President of the company.

[2] He obtained early information in wine cultivation through his father and would often visit the Napa Valley as the Silver Oak business expanded in the 1970s and 1980s, becoming one of California's most successful Cabernet Sauvignon brands.

Duncan paid close attention to sustainability in the design of the new winery, with the desire to create something monumental and an ideal space for making Napa Valley Cabernet Sauvignon and entertaining guests.

[7] In 2007, Duncan purchased the Roshambo Winery in the Russian River Valley near Healdsburg, California with more than 17 acres of vineyards to produce estate wines for Twomey.

[11][12] In 2017, the Duncan family purchased Napa Valley's Ovid winery, with 15 acres of vineyards set in Pritchard Hill.

[14] In December, Duncan was interviewed on CNBC in relation to the Napa Valley's recovery from the wildfires which have affected the region, stating that all of Silver Oak's fruit had been picked before the fires broke out.

In 2016, Silver Oak's Oakville winery became the world’s first production winery to be granted LEED (Leadership in Energy and Environmental Design) Platinum Certification under the Existing Buildings, Operation and Maintenance (EB:OM) rating system – the highest level of certification granted by the U.S. Green Building Council (USGBC) for efficient energy use.

He is also a founding member of a Napa Valley band called the Silverado Pickups and plays the guitar and harmonica alongside other fellow Napa Valley vintners, including Jeff Gargiulo, Shane Soldinger, Dan Zepponi, winemaker Tres Goettings, wine-industry advisor Paul Hoffman, and professional percussionist Joe Shotwell.

[22] Duncan formerly ran a Tumblr blog named "Bottle Stories", in which Silver Oak enthusiasts could post their anecdotes about the brand.