"Duet" is the seventeenth episode of the third season of The CW television series The Flash, which aired on March 21, 2017. The episode was written by Aaron Helbing and Todd Helbing from a story by Greg Berlanti and Andrew Kreisberg, and was directed by Dermott Daniel Downs. The episode features a musical crossover with Supergirl and reunites Grant Gustin and Melissa Benoist with their former Glee co-star Darren Criss, who portrays the Music Meister. Jesse L. Martin also reunites with Jeremy Jordan, with whom he previously worked in the 2012 musical film Joyful Noise. The story begins at the end of the Supergirl episode "Star-Crossed", which aired on March 20, 2017, which saw Kara Danvers / Supergirl (Benoist) hypnotized into a coma by Music Meister, with Mon-El and J'onn J'onzz / Martian Manhunter bringing her to Earth-1 in hope that Team Flash can save her.

Aaron Helbing and Todd Helbing are American sibling screenwriters who had formerly worked as a team. They are known for their work on the Starz series Spartacus and Black Sails, and for The CW superhero drama The Flash. They stopped writing together after Season 3 of The Flash, splitting into separate careers.