Unfortunately RadTreeView doesn't support this kind of functionality, and there isn't any other control in our suite that fits this scenario.
However if you want to highlight the parents of a particular node, you can do that with some client logic. I've attached a small sample page which has some nodes with multiple parents (which is implemented as you said by copying the node in each parent's child collection). When you click a node, all of it's parents are highlighted. You can also modify the code, so that instead of the parents, the nodes themselves are highlighted.
All the best,
the Telerik team
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