Thank you for writing.
There was a undocumented behavior of the Add method that allowed the method to be called with a node that is already in the collection. In the Q2 2008 version that behavior was fixed and it is expected to get an exception if you are trying to add/insert a node that is already in the collection. The Move method should be used if you want to move a node to another position in the same collection.
However, I admit that the exception message was indeed unclear and we have changed it.
the Telerik team
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