overwrite item

[Edneeis]

Private Sub ComboBox1_SelectionChangeCommitted(ByVal sender As Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectionChangeCommitted
        If ComboBox1.SelectedItem Is Nothing Then Exit Sub
        If Not ListBox1.Items.Contains(ComboBox1.SelectedItem) Then
            ListBox1.Items.Add(ComboBox1.SelectedItem)
        End If
    End Sub

If it exists then there is no need to overwrite it since it is the same. If you mean that only one part matches then you'll have to give more information about what you are using as items, otherwise the above should work.

Actually I just reread your post and what I posted is a little bit different. It will add the combo.SelectedItem to the listbox if it does NOT exist in the listbox already.