For the DS9 episode of the same name, please see "Duet".
In music, a duet was a piece performed by two individuals in harmony.
In 2369, Jean-Luc Picard played some duets with Nella Daren, him on his Ressikan flute, her on the piano. Prior to spending time with her, he had never played any before. Daren mentioned this to Doctor Crusher while being treated for a mild strain. (TNG: "Lessons")
In 2375, The Doctor intruded on Tom Paris's The Adventures of Captain Proton holoprogram, having been booked the holodeck to rehearse a duet from the opera Don Carlo. (VOY: "Night")
That same year, while The Doctor tested his auditory nerves by singing, Chakotay quipped, "Pretty soon we'll be singing duets." (VOY: "The Fight")
In 2376, The Doctor told Rudolph Ransom that he and Seven of Nine used to practice duets together, demonstrating with the song "Oh My Darling, Clementine". (VOY: "Equinox, Part II")
Some time prior to 2377, Reginald Barclay sang a duet with Data at Geordi La Forge's birthday party. Deanna Troi reminded him of this as it contrasted sharply with his current demeanor. (VOY: "Inside Man")