The Royals selected Duffey's contract from Triple-A Omaha on Monday.
Kansas City had a spot on its 40-man roster open for Duffey after Jordan Lyles (personal) was placed on the inactive list over the weekend. Duffey, who made just one appearance at the big-league level in 2023 as a member of the Cubs, posted a 3.24 ERA and 1.36 WHIP over 8.1 innings across six relief appearances for Omaha this season.