Instance, Session and Concurrency for duplex service

Question

I have a requirement where a service should be sending a message to client every second. There can be only one client to the service.

I have a created a duplex service and now confused on instance, concurrency and session for the service.

1. Is session always required for a duplex service? SessionMode.Required

2. Does the InstanceContextMode should always be PerSession. InstanceContextMode = InstanceContextMode.PerSession?

3. What should be the ConcurrencyMode?

4. How can I prevent additional client connection to the service. Should the callback reference in teh client be static and check for null reference when client request is initially received.

5. I’ll be implementing a heartbeat operation to check the availablity of the service. Should this be a one-way or request reply call and would this call be in same session?

Thanks.