Kirihiko's little sister Yukie/Yuko in "Y's Tragedy" is Kaoru Hirata, who played Yuka Yamazaki, also in Mahou Sentai Magiranger.
The human form of Arms Dopant? That's AbaRed (Kouichirou Nishi). As if to push the point further, he states repeatedly that he loves to "rampage"note "Abare". And on top of that, Shotaro and Philip fight him with what is basically the Double equivalent of Abarekiller, spines and all.
And HurricaneBlue (Nao Nagasawa) appears in "The D was Seen" as Lily Shirogane, a stage magician who uses a Dopant Memory to cheat at her disappearing act. She would return in the Accel edition of Kamen Rider Double Returns as a waitress at her grandfather's coffee shop. She is still a magician, though, and even manages to allow Shotaro and Philip to escape police custody via her Memory-free vanishing act so they can assist Accel.
Amane Kurihara from Kamen Rider Blade (Hikari Kajiwara) shows up in "The Vengeful V" as one of the art students.
The actor playing the human form of the Liar Dopant was in Kamen Rider Kabuto, playing a surgeon that rotated his duties with two Worms mimicking him.
Kirihiko showed up way back in the first two episode of Den-O as one of the bullies who harassed Ryotaro.
Of all the Japanese comedians who get bit parts in this show, the human form of Cockroach Dopant is a member of the comedy group Rahmens, best known for his role as "Persocom" from the Japanese Mac and PC commercials.
The teacher from "Find the C" which accosts the Dance Battler Dango could very well help Double with his crime-solving exploits as he is, after all, a retired Metal Hero.
Name's the Same: The show's songs have this, but they're named after some Disney songs: Nobody's Perfect and Naturally. The former song has two: One of a upbeat song sung by Hannah Montana, the other is a gloomy one by Koji Kikkawa. The latter one: One's an upbeat song by Selena Gomez (and the Scene), the other sung by Wakana Sonozaki. Double points: It helps that Avex, the one making songs for the Kamen Rider Franchise since 2001, distributes The Walt Disney Records/Hollywood Records label in Japan, so the titles are (maybe) coincidence.
While fans have been clammoring for FangMetal or FangTrigger since the Fang Memory debuted, the Double novel explains why these forms can't ever be used. While the Fang Memory normally turns Philip into The Berserker, Shotaro's compatibility with the Joker Memory allows him to keep this in check, rendering FangJoker stable. Shotaro has no special compatibility with the Metal and Trigger Memories, meaning those forms couldn't be controlled.