Working with multiple bootstrappers

I have an system consisting of toolbar and multiple tools. Every tool is a separate application provided as a set of dll files. During startup system seeks for libraries containing bootstrappers and registers then as icons on toolbar. These applications can be delivered independently and can be run simultaneously.

After upgrade from Caliburn.Micro 1.5.2 to 2.0.2 a problem appeared. When I start application for the first time everything goes fine. However when I start it again from the toolbar it crashes on load with an exception.

System.ArgumentException: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at Caliburn.Micro.AssemblySourceCache.<Install>b__4(Type t)
   at Caliburn.Micro.EnumerableExtensions.Apply[T](IEnumerable`1 enumerable, Action`1 action)
   at Caliburn.Micro.AssemblySourceCache.<Install>b__0(Object s, NotifyCollectionChangedEventArgs e)
   at System.Collections.ObjectModel.ObservableCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
   at Caliburn.Micro.BindableCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
   at Caliburn.Micro.BindableCollection`1.<>c__DisplayClasse.<AddRange>b__d()
   at Caliburn.Micro.XamlPlatformProvider.OnUIThread(Action action)
   at Caliburn.Micro.BindableCollection`1.AddRange(IEnumerable`1 items)
   at Caliburn.Micro.BootstrapperBase.StartRuntime()
   at Caliburn.Micro.BootstrapperBase.Initialize()
   at DemoApp.Starter.DemoUIStarter.Start()
   at ...

What is the way to solve this issue? Is there any solution to run applications with multiple bootstrappers safely?

[aplocher]

I began getting this exception, but after reading this post I remembered that I commented out some old code (to see if it was still needed) but forgot about it, because I was only testing with one bootstrapper for the longest time. Once I tried a second one (weeks later) I ran into this. Uncommented my old code (below) and works great again lol.

protected override IEnumerable<Assembly> SelectAssemblies()
{
    //http://www.andyrace.com/2014/05/caliburn-micro-and-usercontrols.html
    return base.SelectAssemblies().Where(x => !AssemblySource.Instance.Contains(x));
}

Thank you

[nigel-sampson]

@aplocher nice simple fix, thanks