Workaround:
 Private Sub RadDiagram1_MouseDoubleClick(sender As Object, e As MouseEventArgs)
        If e.Clicks = 2 Then
            Dim shape As RadDiagramShape = Me.RadDiagram1.DiagramElement.ElementTree.GetElementAtPoint(e.Location).FindAncestor(Of RadDiagramShape)()
            If shape IsNot Nothing Then
                'ToDo
            End If
        End If
    End Sub