The Indians came back in the bottom of the eighth when Asdrubal Cabrera and Carlos Santana led off with back-to-back bloop singles against Matt Thornton (1-2), and one out later Duncan ripped a pitch into the left-field corner for an RBI double.
Tony Sipp tossed a scoreless ninth to pick up his second career save and the American League Central-leading Indians took their sixth win in seven contests after waiting out a 90-minute rain delay.
Cleveland won the opener 8-6.
2014: NFL Cheerleaders [PHOTOS]
Michael Sam cut from the Dallas Cowboys