[1] After retiring as a player, Duncan served as the pitching coach for the Indians, Seattle Mariners, Chicago White Sox, Athletics, and St. Louis Cardinals.

Duncan was signed as an amateur free agent by the Kansas City Athletics in 1963, as a 17-year-old out of Crawford High School in San Diego.

[2] In his first at-bat as a professional baseball player, he hit a home run for the Daytona Beach Islanders of the Florida State League.

A 19-year-old sophomore center fielder with Arizona State, Monday had led the Sun Devils to the 1965 College World Series title on June 12 and was the first pick in the first major league draft four days earlier.

[11][12] When his hitting showed signs of improvement, Duncan returned to the major leagues in September, along with Jackson and Joe Rudi.

[18] When Duncan made adverse comments about Finley during the season, the owner fired Athletics manager John McNamara in October for failing to control his players, despite the team's second-place finish in the American League Western Division.

[19] The 1971 season saw Duncan become the Athletics main catcher, as he guided their pitching staff to finish second in the league in earned run average as well as in strikeouts.

[20] His offense continued to improve, hitting 10 home runs by mid-season and, was selected as a reserve on the 1971 American League All-Star team, replacing Ray Fosse who missed the game due to an injury.

[27] Despite Duncan's production, Athletics manager Dick Williams, a proponent of the platoon system, began using Gene Tenace as the starting catcher during the last month of the season.

[1] Duncan was acquired along with minor league outfielder Alvin McGrew by the Baltimore Orioles from the Indians for Boog Powell and Don Hood on February 25 1975.

After a stint as a pitching coach for the Seattle Mariners in 1982, he joined former teammate La Russa, then the manager of the Chicago White Sox.

Beginning in 1986, first base coach Dave McKay also began a long tenure of working with Duncan and La Russa.

Pitchers on Duncan's staffs won four Cy Young Awards: LaMarr Hoyt in 1983; Bob Welch in 1990; Dennis Eckersley in 1992; and Chris Carpenter in 2005.

[41] Dave Stewart, who had not found consistent success before signing with Oakland as a free agent in 1986, won 20 or more games and pitched 250 or more innings four straight seasons from 1987 to 1990.

[46] On November 13, 2013, the Arizona Diamondbacks announced they had hired Duncan to be a special assistant to general manager Kevin Towers as a pitching consultant.