Duncan is the sole shareholder of the firm whose practice primarily focuses on estate planning, trusts, business matters, and construction law. He graduated from Southern Methodist University in 1977 with a business administration degree in accounting and finance and earned his law degree from SMU in 1980.
Duncan has more than thirty-five years of legal experience and has been admitted to practice in the United States Tax Court, United States District Court for the Northern District of Texas, and Fifth Circuit of the United States Court of Appeals.
During the three years Inside Collin County Business conducted its annual awards, Duncan was voted “Best Business Lawyer” in Collin County all three years (1997-1999) and “Best Business Tax Consultant” one year (1997). Duncan was also voted "Best Attorney in Plano/Frisco" by Living Magazine for 2016 and 2017.
Duncan is a Sustaining Fellow of the Texas Bar Foundation and Collin County Bar Foundation. Duncan served on the District 1A Grievance Committee of the State Bar of Texas for six years and served as its Chairperson in 2006–2007. Duncan is a member of the State Bar of Texas, Collin County Bar Association, and the Plano Bar Association.
Duncan was elected and has been serving since January 1, 2011 as Collin County's Commissioner for Precinct 4. Duncan also serves on the Regional Transportation Council and the Dallas Regional Mobility Council.
Duncan has previously served on the boards of directors of the Medical City Plano, Assistance Center of Collin County, the Plano Library Advisory Board, The Classics, the Plano Family YMCA, and the Leadership Plano Advisory Board. Duncan and his family are members of the Christ United Methodist Church.
Duncan and his family have been recognized for their family values and service to their community by being named as Plano’s Family of the Year in 1997 by the Plano Family YMCA and as Citizens of the Year in 2008 by the Plano Chamber of Commerce.