Forms :: Node Displayed Multiple Times In Treeview Control?

I'm developing a windows application. I have a form in which there is a list box and a treeview control. My purpose is to fill treeview according to list item clicks. This is done using a pair of subroutines called ReloadPermissions and FillMenuItem:

Private Sub ReloadPermissions()
Dim p As New Permissions
Dim i As Integer
Dim it As TreeNode
Dim el As New List(Of String)

[Code]...

There are only two subroutines in which nodes are added to treeview control, and they are subroutines above. The problem is that whenever list item is clicked, node count displayed in treeview is increased!!! I checked number of nodes before and after call to tvPermissions.Nodes.Clear() using MsgBox. Results show that number of nodes isn't increased (1 before call, 0 after call, again 1 after ReloadPermissions() is called), but what is displayed doesn't show such a behavior! Refreshing window by putting it under other windows or call to tvPermissions.Refresh() has no effect.