DiagnosticsTelemetryModule can get added more than once

[pharring]

Repro Steps

1. Compile and run:

using System;
using Microsoft.ApplicationInsights;
using Microsoft.ApplicationInsights.Extensibility;
using Microsoft.ApplicationInsights.Extensibility.Implementation;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            TelemetryConfiguration.Active.Dispose();
            new TelemetryClient();
            Console.WriteLine(TelemetryModules.Instance.Modules.Count);
        }
    }
}

Actual Behavior

Outputs: 2

Expected Behavior

Outputs: 1

Version Info

SDK Version : 2.10.0-beta3
.NET Version : net46, netcoreapp20
How Application was onboarded with SDK(VisualStudio/StatusMonitor/Azure Extension) : VisualStudio
OS : Windows 10
Hosting Info (IIS/Azure WebApps/ etc) : N/A

[pharring]

What's going on here is that the DiagnosticsTelemetryModule is automatically injected into TelemetryConfiguration.Active by the TelemetryConfigurationFactory.
That module is held in a static instance of TelemetryModules. If you dispose of TelemetryConfiguration.Active, and re-create it (e.g. by creating a new TelemetryClient), then another instance of DiagnosticsTelemetryModule is added. Once that happens, you've got multiple EventListeners each hooked up to a PortalDiagnosticsSender which leads to duplicate telemetry being sent for each diagnostics event.

The key here is calling Dispose on TelemetryConfiguration.Active allowing it to be re-created. That happens in unit tests. It's probably unlikely in customer code, but not impossible (as I've shown here).