HttpHandlerDiagnosticListener duplicate event repro

Program.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Net.Http;
using System.Net;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;

class Program
{
    private const string RemoteEchoServer = 
        "http://corefx-net-http11.azurewebsites.net/Echo.ashx";
    //  "http://example.com"; // Other hosts work well

    static async Task Main()
    {
        var eventRecords = new EventObserverAndRecorder();

        // ActivityIdFormat.Hierarchical works well
        Activity.DefaultIdFormat = ActivityIdFormat.W3C;

        // HttpWebRequest works well
        using (var client = new HttpClient())
        using (HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get, RemoteEchoServer))
        {
            (await client.SendAsync(request, default)).Dispose();
        }
        
        // PRINTS:
        // System.Net.Http.Desktop.HttpRequestOut.Start: HttpWebRequest[6044116] -- http://corefx-net-http11.azurewebsites.net/Echo.ashx
        // System.Net.Http.Desktop.HttpRequestOut.Start: HttpWebRequest[6044116] -- http://corefx-net-http11.azurewebsites.net/Echo.ashx
        // System.Net.Http.Desktop.HttpRequestOut.Stop: HttpWebRequest[6044116] -- http://corefx-net-http11.azurewebsites.net/Echo.ashx
    }
}

internal class CallbackObserver<T> : IObserver<T>
{
    public CallbackObserver(Action<T> callback) { _callback = callback; }
    public void OnCompleted() { }
    public void OnError(Exception error) { }
    public void OnNext(T value) { _callback(value); }

    private Action<T> _callback;
}


internal class EventObserverAndRecorder : IObserver<KeyValuePair<string, object>>
{
    public EventObserverAndRecorder()
    {
        DiagnosticListener.AllListeners.Subscribe(new CallbackObserver<DiagnosticListener>(diagnosticListener =>
        {
            if (diagnosticListener.Name == "System.Net.Http.Desktop")
            {
                diagnosticListener.Subscribe(this);
            }
        }));
    }

    public void OnCompleted() { }
    public void OnError(Exception error) { }

    public void OnNext(KeyValuePair<string, object> record)
    {
        HttpWebRequest r = (HttpWebRequest)record.Value.GetType().GetProperty("Request").GetValue(record.Value);
        Console.WriteLine($"{record.Key}: HttpWebRequest[{RuntimeHelpers.GetHashCode(r)}] -- {r.RequestUri}");
    }
}