Duplex Service in WCF & Silverlight with C#

Question

Hi,

        I want to develop client-server web application using wcf service in silverlight c#.
Help me how to create duplex service to send and receive data with particular specified client.

        Its only one way communication to pass a message to a single client.
i.e.
        if 100 users are online, server can send a message to all or any single user by this.

Need Help.!