How can a WCF Service Raise Events to its Clients?

There is no Event in WCF to notify it’s client but there is a callback channel, the purpose of the callback channel is same as event though the working principle is totally different in both cases.
To notify a particular client what you could do is store callback channel of that client while subscribing to somewhere, (I prefer Dictionary in this case). Later you can pick the instance and invoke your callback method over that channel, doing so only one client will get notified.

UPDATE

If you are interested here is the code:

public interface IClientCallback
{
    //Your callback method
    [OperationContract(IsOneWay = true)]
    void CallToMyClient(string name);
}
[ServiceContract(CallbackContract = typeof(IClientCallback))]
public interface ITestService
{

    [OperationContract(IsOneWay = true)]
    void SubscribeClient();
    [OperationContract(IsOneWay = true)]
    void DoSomeWork();
}
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall, ConcurrencyMode = ConcurrencyMode.Reentrant)]
public class ServiceImplementation : ITestService
{
    private static readonly List<IClientCallback> CallbackChannels = new List<IClientCallback>();

    /// <summary>
    /// client should call this method before being notified to some event
    /// </summary>
    public void SubscribeClient()
    {
        var channel = OperationContext.Current.GetCallbackChannel<IClientCallback>();
        if (!CallbackChannels.Contains(channel)) //if CallbackChannels not contain current one.
        {
            CallbackChannels.Add(channel);
        }
    }

    public void DoSomeWork()
    {
        //Here write your code to do some actual work
        //After you done with your work notify client
        //here you are calling only the first client that is registered
        IClientCallback callbackChannel = CallbackChannels[0];
        callbackChannel.CallToMyClient("You are the only one receving this message");
    }
}