Tom Cavanagh isn't the only one to pull ArrowVerse double duty.
Plenty of actors on The Flash have played multiple characters, including multiverse doppelgangers and villainous lookalikes. But the man who plays Captain Singh on The Flash has an Arrowverse history that's less obvious than his castmates. Patrick Sabongui first appeared as a low-level drug dealer in the Arrow Season One episode "Vertigo" and again, as a random soldier, in Episode 17, "The Huntress Returns."
The Prison Break connection.
Wentworth Miller's portrayal of Leonard Snart was an immediate breakout (pun intended) on The Flash and it was a stroke of brilliance on the part of the show's producers to bring in Miller's old Prison Break costar, Dominic Purcell, to play Snart's partner-in-crime Mick Rory, aka Heatwave. This onscreen reunion actually led to the revival of their old show. As Miller said in a 2017 interview, "We hadn't seen each other in about five years. Suddenly we were back on set, discussing old times. And out of that conversation came the possibility of revisiting Prison Break."
The Big Bang Theory connection.
Sheldon Cooper is a well-known fan of The Flash and Cisco Ramon appears to be a fan of The Big Bang Theory. There have been several nods to the massive CBS comedy in the form of Cisco's t-shirts, which have included, "That's my spot," "Rock/Paper/Scissors/Lizard/Spock" and most notably, "Bazinga." (No word on whether or not any of the multiverse versions of Vibe are fans of Young Sheldon.)
The Candyman connection.
Zoom, the Big Bad of The Flash Season Two, was played by actor Teddy Sears but voiced by Tony Todd, veteran character actor from The Crow and Star Trek: The Next Generation, best known as The Candyman. Todd portrayed the supernatural killer in the 1992 horror film and its two sequels, Farewell to the Flesh and Day of the Dead.
The Glee Connection.
Of course a musical episode was inevitable for the Arrowverse! Both Grant Gustin and Melissa Benoist were part of the Fox musical comedy Glee. The Flash/Supergirl crossover, which gave us the best musical episode since Buffy The Vampire Slayer, reunited both actors with Darren Criss, who joined the Arrowverse as Music Meister. Who knows? Maybe Glee takes place on one of The Flash's multiple Earths.