The veteran infielder singled in the ninth inning to become the second member of the Cardinals to hit for the cycle this season as Double-A Springfield cruised to a 6-2 victory over the Arkansas Travelers.
"It's a very neat feeling, pretty special," Duncan said. "It's something I'll savor tonight, talk about it with my family."
The 26-year-old New Jersey native flied out in the first inning, leaving him in a 3-for-20 slump. But he broke out with a double in the third and scored on a two-run single by former first-round Draft pick Zack Cox.
Duncan led off the fifth with his career-best 22nd homer and got the hardest part of the cycle out of the way when he tripled in the seventh and came home on Cox's sacrifice fly.
"I believe that was my first one of the year," said Duncan, who had a career-high five last season but none since Sept. 3. "We were playing at a pretty spacious ballpark. I was fortunate enough to hit in a spot, the gap, and get a triple."
Was he thinking three bases right out of the box?
"Not necessarily," Duncan said. "I was thinking, 'Run hard' once I saw it get down. Halfway between first and second, I was thinking I would try to go for this."
He made it, leaving him a single away from something no one in team history had accomplished before this season.
"I was definitely aware of it, I definitely knew what was happening," he said. "I don't think hits are ever easy. I was trying to put together a good at-bat against a good arm like [David] Carpenter."
After taking two strikes and a ball, Duncan lined a base hit up the middle for his first four-hit game since going 5-for-5 for Double-A Mississippi on May 31, 2010.
"I was just trying to lay off something away and try to fight off pitches. Hopefully, he'll make a mistake and I'll try to get back in the at-bat," he explained.
Duncan was promptly lifted for a pinch-runner after becoming the second Cardinal to hit for the cycle in less than three months. Domnit Bolivar was the first on June 10.
"It just shows you how crazy this game can be," said Duncan, the Yankees' first-round pick in the 2003 Draft. "It's a humbling game, but it also can be very rewarding."
Thanks to Duncan and Cox, Shelby Miller (8-3) was rewarded with his first win since Aug. 4. The Cardinals' top prospect gave up a run on four hits and a walk while striking out six over five innings.