When value types are inserted in the Items collection of RadTreeView and root values are duplicated in the children a StackOverflowException is raised after a call to ExpandAll.

The XAML has recently reviewed the status of this issue and won't be addressing it in future releases.
This is a design limitation - RadTreeView does not support duplication of value types in its Items / ItemsSource collections. 
This potentially can cause troubles with virtualization / selection / expandALL features.