Matt Bonner added 15 points, going 3-for-3 from 3-point range and 6-for-7 overall from the field, for the Spurs (2-2).
Tony Parker had 14 points and five assists after missing the Spurs' game Tuesday night against Detroit because of a sore left Achilles' tendon. He played 17 minutes in the first half, then sat out the second half.
Even though it was an exhibition game, Duncan, who was 6-of-10 from the field and 4-of-4 from the line, showed midseason form.
"Another double-double," Spurs center Kurt Thomas said. "You couldn't ask for any more than what he does for us."
"This was the first night we played him into the third quarter," San Antonio coach Greg Popovich said. "Next week, we will get him into the fourth quarter so he's ready for the season. He's looking for his offense and shooting the ball. He's doing it from the outside with his shot, not just down on the block."
LeBron James scored six points in 17 first-half minutes for the Cavaliers. He had two rebounds, five assists and a blocked shot.
James, who didn't play in the second half, isn't concerned that the Cavaliers haven't won a game in the preseason.
"None of our big guys are playing in the fourth quarter when it's closing time," James said. "Once we get to the last couple preseason games and start playing some more minutes, we'll try to get a win."
Cavaliers coach Mike Brown will begin playing his starters more minutes in Saturday's game against Philadelphia.
"Most likely, I'll play our starters in the second half," Brown said. "I haven't really done that yet. I've been trying to give a lot of guys an opportunity."
Zydrunas Ilgauskas scored 10 points in 22 minutes.
Daniel Gibson started at shooting guard for Cleveland. Delonte West, who will see playing time at both guard spots for the Cavaliers, played in his first game. He missed 10 days of training camp because of personal reasons before returning Wednesday.
West scored seven points on 2-of-12 from the field in 28 minutes.
"I wanted to get him on the floor just to let him sweat and run and get used to being in the game," Brown said.
Mo Williams, a key offseason pickup by the Cavaliers, was 3-of-14 from the field and scored eight points.
James was honored in a halftime ceremony for being a member of the U.S. Olympic championship team.