The Spurs have recorded 29 straight home victories over the Warriors, who could not make a significant charge after falling behind by 13 at halftime.
Duncan also had 13 rebounds and six assists. San Antonio won its third in a row and has 52 victories for the year, one less than Miami for the league lead. Tiago Splitter contributed 17 points and Manu Ginobili had 16.
San Antonio has a 6-2 record since point guard Tony Parker suffered an ankle sprain that has him out indefinitely.
Golden State, which had won four out of five, was led by 24 points from Stephen Curry.