Need Help Updating Items in a ListBox

[pythonregius24]

        If strCustomerOrder = "A1#" Then
            decSubtotal = ANTACID_ONE
            mintQuantity += 1
            ShoppingCartForm.lstShoppingCart.Items.Add(("     A1" & vbTab & vbTab & "Alka-Seltzer Antacid" _
                                                       & vbTab & vbTab & mintQuantity & vbTab _
                                                       & "     $" & ANTACID_ONE).ToString())
            BuyItemSoundEffect()
            lblKeypadScreen.Text = String.Empty

Need Help! I want my listbox to not display duplicate items. I just want it to update the quantity(Qty.) I need help figuring this out. Any suggestions?