He's always been protected against LHP, he's got relatively few ABs against them over the course of the 3 years he's appeared in the majors. And he's 26, so most of his development/learning has already taken place.
There's really nothing wrong with sitting a guy against his same-side platoon. Cora is merely maximizing his effectiveness. And it's working.
Duran has been so great this year that it's very tempting to just play him every day. But you're right - he's been hugely successful being put in a position to succeed (facing RHP). Which is awesome, and the results have been fantastic.
Here are his splits:
vs RHP: .325/.376/.532/.908
vs LHP: .278/.325/.417/.742
I mean, it's not like he's been BAD against lefties. He's just mashed righties. I think he would be ok playing every day, but without question his overall numbers would be doing down.
One of the questions is: who would be better against LHP: Duran or Refsnyder? And the answer is obvious.
vs LHP:
Duran: .278/.325/.417/.742
Refsnyder: .346/.463/.449/.912
So if you platoon those two, you're getting a .910 ops player in the outfield, which is pretty awesome.
I know Refsnyder doesn't play CF. But I think (if I've been following these moves correctly) that Refsnyder tends to play when Duran doesn't, and they make the appropriate defensive shift.