Any song can be used for a duet--you simply have to arrange it as a duet.-There are a bunch of the duet tricks that you can use--stuff like alternating lines, which turns any song into a dialog--singing some lines or verses in unison, and the refrain in harmony--adding a countermelody to the refrain on the second or third repetition--(and there are five different species of counterpoint--so you never need to fall into a rut)--

Don't forget that there are also a number of ways of harmonizing, too--the harmonic line can follow the melody, or it can change only as the chords change--it can even not follow the chord changes, which would make it a drone. You can even have the harmony voice improvise against a fixed melody--

Really though, all you really need to create a duet is to have two people sing together--