Duklyon In Anime

Gakuen Tokkei Duklyon was never animated, but the characters do appear in the CLAMP Gakuen Tanteidan anime. Kentarou was voiced by Kappei Yamaguchi and Takeshi was voiced by Kusao Takeshi, appropriately enough. Erii was voiced by Hidaka Noriko.

AnimeNewsNetwork's CLAMP Gakuen Tanteidan page can be found here. CLAMP Gakuen Tanteidan was brought out in North America by AnimeVillage.com, but it's remarkably hard to find it now. It is available on the internet if you know where to look, though.

I'll have more in this section once I've seen the rest of the series!

CLAMP in Wonderland

CLAMP in Wonderland feels a bit like a predecessor to Tsubasa, in that it's a giant mix-up of CLAMP characters. Unlike Tsubasa, though, CLAMP in Wonderland isn't a manga, but rather a short animated music video set to the song "Anata Dake no Wonderland" by Hirotani Junko. The music video came out in 1994 and features all of CLAMP's characters from their earlier manga, including Duklyon (pictures open in a new window):

CLAMP In Wonderland has never been released in North America, but it is available online if you can find it.

Tsubasa Chronicles

Tsubasa Chronicles is the anime counterpart of Tsubasa RESERVoir CHRoNiCLES. Although the plot has been noticeably changed for the anime, it does offer more animated Duklyon in episode 18. The scene is fairly close to the one from the manga, but it's quite a bit shorter, and neither Kentarou nor Takeshi talk. Erii is voiced by Hayashi Marika. All pictures open in a new window.