Dear Old Friend, from "Love Never Dies," is one I like. I can't remember whether it's actually a duet or a trio, but it can easily be sung as a duet.
Matchmaker, Matchmaker can probably be done as a duet.
I think "If/Then" has one or two.
Audrey
Audrey, the Phantom Phanatic, who nonetheless would rather be Jean Valjean, who knew how to make lemonade out of lemons.