He's "STARTED" a total of 8 games since he came over - that means that 30 of his ABs have come when he's seen more than 1 in the same day, and he's started back-to-back games exactly once. The SPs in those 8 games were...
Jason Vargas
Jefry Rodriguez
Tommy Milone
Wade Miley (shut the entire offense down that day)
Jose Urena (the day he drilled Acuna, Duvall replaced him - assuming you want to consider that one a start)
Kyle Freeland (shut the entire offense down on 3 hits before the pen blew it late)
Wei-Yin Chen (the day he had his best start in years and shut the entire offense down)
Mike Montgomery
He starts almost a game and a half per week, and in half of those games he's faced a no-name SP who has spun one of their best starts in recent memory for crying out loud.
Duvall has had so much less to do with this team losing games than the bullpen since he was acquired it's comical.