I didn't label him a specialist, but note that Duke averaged well below an inning per appearance each of the last 4 years, and he only faced 56% RHB during that time (versus, for example, 65% for Taylor Rogers last year). That suggests Duke's usage had tended towards specialist, and his results vs RHB should be viewed in that light (he's probably not facing many good RHB, although Caleb Joseph isn't a good one either but with 20+ pitches and the go-ahead run at second, that threshold is lowered considerably).