This song from Act I of the blockbuster Andrew Lloyd Webber musical The Phantom of the Opera is performed principally by the heroine/damsel Christine Daaé. In the 2004 film version she begins singing during a rehearsal of an imaginary opera which morphs quickly into the actual performance backed by the full orchestra. Seeing her perform it, a surprised Raoul sings a verse, but as he is not on the stage and his presence is fairly fleeting, this can hardly be called a duet.
Suggestion credit: Alexander Baron - London, England
Sarah Brightman, then Andrew Lloyd Webber's wife, originated the role of Christine Daae in the musical. For the 2004 film adaptation, Emmy Rossum played the heroine and earned a Golden Globe nomination for Best Actress in a musical or comedy.