[ServiceContract(..., CallbackContract = typeof(IDuplexCallbackContract))]
    public interface IDuplexContract
    {
        [OperationContract(...)]
        void Login(Message message);
        [OperationContract(...)]
        void Call(Message message);
        ...
        [OperationContract(...)]
        void Logout(Message message);
    }

    [ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Multiple)]
    public class DuplexService : IDuplexContract
    {
	Dictionary<string, IDuplexCallbackContract> clients ...
        ...
        public void Login(Message message) // регистрация клиента
        {
	   ...
           lock (((ICollection)clients).SyncRoot) {
		clients.Add(OperationContext.Current.Channel.SessionId, OperationContext.Current.GetCallbackChannel<IDuplexCallbackContract>());
           }
	   ...
        ...
        public void Call(Message message) // клиент дергает сервис
        {
           string sessionId = OperationContext.Current.Channel.SessionId;
	   IDuplexCallbackContract ch = null;
           lock (((ICollection)clients).SyncRoot) { // находим "сессию" клиента
                if (clients.ContainsKey(sessionId)) {
                    ch = clients[sessionId];
                }
           }
           if (ch != null) { // работаем с каналом клиента
	   ...