"Young and Foolish" from 1955's PLAIN AND FANCY, from a collection entitled "Broadway Ballads" published by the Hal Leonard Corporation. I'm sure there are collections of Broadway duets, too. "Young and Foolish" is a duet, although the tenor part has more to sing than the soprano. It is a beautiful song, very romantic. I bought the collection at the music store on Broadway and 49th Street(whose name I can't remember).
If you're still taking suggestions, "I've Never Been in Love Before" from "Guys & Dolls" is one of my favorites.
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I find Guys and Dolls is a good source for romantic duets. "I'll Know" "I've Never Been in Love Before" and, from the movie, "Woman in Love" are all good choices in my opinion.
I also like "I Should Tell You" and since this can also probably work with a boy and a girl "I'll Cover You" from RENT.
And, one of my personal favourites at the moment, "Something Good" from the movie version of The Sound of Music. I just love the slow, romantic music and the sweet simplistic lyrics.
I'd Give it All for You (Songs for a New World) Nothing is too Wonderful to Be True (Dirty Rotten Scoundrels) (more comedic really...) Accident Waiting to Happen (Drowsy Chaperone) The Next Ten Minutes (Last Five Years) Goodbye Until Tomorrow/I Will Never Rescue You (Last Five Years) Because of You (I Love you Because) Count on My Love (Lone Star Love) if you can find the sheet music. and there are loads more..
I Love you Because has some great comedic duets...if you are into that. Also..look at Jekyll and Hyde.