What is the main difference between Request-Response and Duplex in WCF Message Exchange Pattern?

The main difference is that, after the client establishes a channel to the service, the service can call the client independently at any time. In Request-Response the service only communicates back after receiving a Request from the client. So by using duplex you receive an event-like behavior from the client perspective. Obviously, such enhancement requires a Session to be maintained (instance mode PerSession on the service).