Duet is a fast-paced game about avoiding shapes through rotation similar in concept to a title like Super Hexagon. You control two circles at the bottom of the screen (red and blue) bound by a ring. Touching left or right rotates the ring accordingly. This technique is used to avoid shapes that appear from the top of the screen, quickly rotating left and right to make sure the red and blue circles do no touch them. Levels are fairly short, but a single hit means you start again from the beginning.
The game is split up in nine sets of levels named after the Kübler-Ross stages of grief and a few other themes. Levels are unlocked gradually and a green icon appears when a level is completed in one go without dying. By completing sets of levels additional modes can be unlocked, such as an endless mode with multiple lives, a daily challenge with different waves, and 48 new stages grouped together as an epilogue chapter. Between levels messages often appear as well as voiced lines.