Duality was a term used to describe someone's emotional state when two personalities were in one person.
In 2364, Counselor Deanna Troi sensed a feeling of duality in Beverly Crusher and Worf after the two were occupied by the Beta Renner cloud. (TNG: "Lonely Among Us")
Two years later, Troi told Captain Jean-Luc Picard about the duality she sensed in Roga Danar. (TNG: "The Hunted")