Is Double Decker! Doug & Kirill: Extra hentai?
No
Rated R (17+ (violence & profanity)) on MyAnimeList and not tagged as Hentai.

Special · 2019 · 3 episodes · R - 17+ (violence & profanity)
Action, Adult Cast, Comedy, Detective, Mystery, Sci-Fi
During the final episode pre-screening event on Friday, it was announced that Double Decker! Doug & Kirill TV anime will get three extra episodes. Titled Double Decker! Doug & Kirill: Extra, the three episodes will be streamed via Bandai Channel and other streaming websites between February and April 2019. The Blu-ray and DVD for the extra episodes will also be released on May 24, 2019. (Source: MAL News)
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