Why does the event fire twice (rx)

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C#

Hi,

I am using Reactive Extensions to handle events fired from a windows service. Here is the DataParser class:

namespace MessageQueue
{
    public class DataParser
    {
        public DataParser()
        {
            var fwLanded = Observable.FromEvent<ThreadQueue.ArrivedHandler, ServiceData>(handler =>
                {
                    ThreadQueue.ArrivedHandler fHandler = (e) =>
                        {//TODO: Fires twice!
                            handler(e);
                        };
                    return fHandler;
                },
                fHandler => ThreadQueue.ArrivedEvent += fHandler,
                fHandler => ThreadQueue.ArrivedEvent -= fHandler
                );

            fwLaoaded.GroupBy(fwl => fwl.Instance).Subscribe(g =>
                {
                    LoadedParser landedParser = new LoadedParser(g.Key);
                    g.Subscribe(grouped =>
                        loadedParser.Add(grouped.tracker, grouped.trackerInstance)
                        );
                }
                );
        }
    }
}

The ThreadQueue is all static so when the service starts I instantiate this class and nothing more.

The event is fired from the queue class here:

public class DataEventArgs : EventArgs
{
    public ServiceData ServiceData;
}
public delegate void ArrivedHandler(ServiceData data);
public static event ArrivedHandler ArrivedEvent;

private static void OnCheck(ServiceData data)
{
    if (ArrivedEvent != null)
        ArrivedEvent(data);
}

private static void ProcessMessage(Message message, int threadIndex)
{
    message.MarkStarted();
    Monitor fm = new Monitor(message.trackerInstanceId);
    ServiceData data = fm.Check();
    if (data != null)
    {
        if (data.IsTracked)
        {
            if (data.IsLoaded)
                OnCheck(data);
            message.MarkComplete();
        }
        else
        {
            message.MarkComplete();
            RequeueMessage(message, 10);
            //TODO: found mismatch!
        }
    }
    else
    {
        RequeueMessage(message, 10);
    }
    _waitHandles[threadIndex].Set();
}

(Forgive the var & method naming. I have cut words to hide the nature of the project.)

I have debugged the code. Each message is handles once (no accidentally picking up the same message twice); the ThreadQueue ProcessMessage is run once per message, and the ArrivedEvent in OnCheck is called once per message. And yet, here is the problem

fHandler fires twice >_<
I cannot see why. I am currently checking the database to avoid inserting duplicates but I will need to add duplicates when multiple trackers are in place so I can record what each tracker has found in case they disagree (trackers are third party).

Any help would be appreciated. I have several mock projects so I can try any suggestions and / or post results and amendments

Thanks