WCF Single Service with multiple clients

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WCF

WCFServices

I have a single WCF service with multiple clients. This is implemented using Duplex communication.
I have a request for execute a command(request from client) and send back the result to the same client.

My InstanceContextMode is PerSession.

Currently Result is getting updated to all the client. I need to just update the send to the client which triggered execute. How to achieve this?

Sample Service Code:

ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
public class NotificationService : INotificationService
{
    
static Action<string> sendDataEvent = delegate { };

private INotificationServiceCallBack GetCurrentCallback()
        {
            return OperationContext.Current.GetCallbackChannel<INotificationServiceCallBack>();
        }


 public void Subscribe()
        {
            var subscriber = GetCurrentCallback();
            sendDataEvent += subscriber.SendResult;
        }


        public void Execute()
        {
           
                //logic and then send the result back
                sendDataEvent(measurements);
       }
}

Client Code:

public class TestDataInterface:INotificationServiceCallback,INotifyPropertyChanged
   {
static Dictionary<string, TestDataInterface> instance=null;
 string testBench=string.Empty;

        NotificationService.NotificationServiceClient notificationProxy;


private TestDataInterface(string testBenchName)
       {
           testBench = testBenchName;
           notificationProxy = new NotificationService.NotificationServiceClient(new InstanceContext(this));
           notificationProxy.Subscribe();
       }

public static TestDataInterface Instance(string testBenchName)
        {
            if (instance == null)
                instance = new Dictionary<string, TestDataInterface>();
            //Add object only if key testBenchName is not found
            if (!instance.ContainsKey(testBenchName))
            {
                instance.Add(testBenchName, new TestDataInterface(testBenchName));
            }
            return instance[testBenchName];
        }

#region INotificationServiceCallback Methods
        public void SendResult(string testResult)
        {
            //logic
        }
        #endregion

public void ExecuteCommand(string command)
       {
                      notificationProxy.Execute(command);
       }