Duke's debut month in July 2005 included a 3–0 shutout victory against Greg Maddux and the Cubs on July 16 and 22 consecutive scoreless innings from July 2 to 21. He was named National League Rookie of the month for July while compiling a 0.87 ERA, the best among all starting pitchers in the Major Leagues. He became only the second Pittsburgh rookie to win his first 5 decisions, along with Whitey Glazner. He also became one of only four pitchers during the Live-ball era to record an ERA below 1.00 in their first six starts (the others being Fernando Valenzuela, Boo Ferriss, and Steve Rogers). Duke finished 2005 with an 8–2 record in 14 starts, striking out 58 in 84.2 innings. He finished in fifth place in the Rookie of the Year voting, garnering 10% of the vote.