A Duplex transition exists through defining a connection from a landing on both feet, to a takeoff from both feet. There is only one natural form, involving no change in position or stance in between landing and takeoff.
Common Duplex Transition
The Punch transition is perhaps the easiest to identify; as the only natural Duplex transition, it represents a default connection between landing and takeoff using both feet simultaneously. A blanket term "Bound" can be used to describe artificial forms which allow a change in position (but still the same connections,) as the Duplex category continues to be explored.