Application crashes if scanned assemblies are duplicated

We have a plugin model that discovers view/viewmodels from external assemblies. If by mistake the assemblies are copied twice (to different discovery locations), the application would blow up with the following stack trace.

The SelectedAssemblies() method uses assemblies where as underlying cache which is the cause of the problem uses the Type name as the key.

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 Caliburn.Micro.EnumerableExtensions.Apply[T](IEnumerable`1 enumerable, Action`1 action)
   at Caliburn.Micro.AssemblySourceCache.<>c.<Install>b__3_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.OnUIThread(Action action)
   at Caliburn.Micro.BootstrapperBase.StartRuntime()
   at Caliburn.Micro.BootstrapperBase.Initialize()
   at ServiceInsight.Startup.AppBootstrapper..ctor() in /_/src/ServiceInsight/Startup/AppBootstrapper.cs:line 41

A simple fix would be to not add duplicate items to the cache if they already exist.

[vb2ae]

@HEskandari thanks for the PR

[HEskandari]

@KasperSK Raised a PR with a failing test and the fix.