I'd say the reasoning might be: pitching Duran in the 8th gives you the best chance of there being a bottom of the 9th. With the bonus if he has a quick 8th you might also get him for the 9th. If you give up a run in the 8th, there's a strong likelihood there ISN'T a bottom 9, and now you've lost a close game late and your best reliever spent the entire game sitting on his ***.
As an aside, Duffey HASNT been better lately. He's had some better results, but not due to better pitching, just better luck.
He fools virtually nobody. He gets virtually zero bad swings. Almost every swing against him is powerful and well timed. He's not reliable, no matter if he's had a couple scoreless innings recently.