How did the Royals decide?
"We determined it the exact same way they determined it," Yost said. "Which game do you want to throw, James? Which game do you want to throw, Justin?"
Both Duffy and Alvarez are being called up from the Minors as the 26th player added to the roster on days that a doubleheader is played.
Duffy made his first Major League start since Tommy John surgery on Aug. 7 against the Twins with a no-decision, then pitched one inning for Triple-A Omaha on Monday.
"It was just a little brush-up on everything," he said. "I feel great."
Duffy's previous brief stay with the hot Royals this season was a revelation.
"I've been around for two days of it and it's definitely a change from years past, so I'm excited to do my part, whatever that may be," he said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.