Stef Smith added 10 points for Vermont, including two free throws that capped the scoring with six seconds left.
Duncan hit a 3-pointer that capped a 9-2 opening run and Lamb's 3 with 9:04 left in the first half pushed the lead into double figures at 18-8. The Catamounts (6-3) led by as many as 14 points in the second half and Duncan's foul shot with 2:56 to play made it 61-49 but Justin Kier scored 14 points from there, including a layup with seven seconds remaining, to pull George Mason within three before Smith's free throws sealed it.
Kier finished with a career-high 32 points on 12-of-15 shooting and Ian Boyd scored 12 for the Patriots (4-6).
Vermont shot 65 percent (15 of 23) from the field in the second half.