Explanation
To create a duplex contract
* (C) Create the interface that makes up the server side of the duplex contract.
* (E) Apply the ServiceContractAttribute class to the interface.
* Declare the method signatures in the interface.
* (E) Apply the OperationContractAttribute class to each method signature that must be part of the public
contract.
* Create the callback interface that defines the set of operations that the service can invoke on the client.
* Declare the method signatures in the callback interface.
* Apply the OperationContractAttribute class to each method signature that must be part of the public
contract.
* (F) Link the two interfaces into a duplex contract by setting the CallbackContract property in the
primary interface to the type of the callback interface.
Reference: How to: Create a Duplex Contract