Hello Dr. Hartmut Kocher,
You are correct when you said that RadTabControl will create only one instance of you UserControl. This is by design.
The place where you add the items from your ViewModel to the SelectedItems of the GridView has to be called within a dispatcher.
sender, DependencyPropertyChangedEventArgs e)
Department department = e.NewValue
I am attaching my sample project for further reference. Have a look at it and let me know if you find something unclear that needs explanation. I'd be glad to further assist you.
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