As I understand it, it was Elvis' ongoing interest in folk music and harmonies during the 60's which led to an eagerness to work with similar textures himself, thus he decided to try 'The First Time Ever I Saw Your Face' as a duet with regular backing singer Ginger Holladay during the first studio sessions of 1971.
When the duet didn't come off the way Elvis' had wanted it, the backing track was put asside for future use.
At a later session, Temple Riser was also given a shot at recording vocals for the duet.
In the end Elvis was to drop the duet approach and make some vocal repairs to make it a traditional solo performance.
Below picture taken at American Studios, Memphis 1969 (Mary Holladay, Jeannie Greene, Elvis, Donna Thatcher, Ginger Holladay.