Duels was a feature on the iOS app that allowed users to compete against each other (or a computer) while also learning a new language. This feature was discontinued because staff "tested it and it hurt engagement, unfortunately".
Duels was a feature that allows users with connected GameCenter accounts (or a user against Duobot) to go head-to-head, translating sentences from written text, pick-and-choose, and listening exercises. One point is earned for a correct answer, and an additional point if one is the first to submit an answer. The feature is currently unavailable, since version 4.0.5.
Users must first complete some lessons within the tree before being able to Duel. Once they have the relevant skills done, they can choose between two modes (Duel a human, or Duel a robot). Each duel has about 10 questions, and users must answer these questions correctly in order to get the most points (which is later converted to XP).