Shopping cart question! Pls help!

Hi.

Can someone pls help me with a little problem I have? I have wrote a code to add items to my shopping cart (code below). What I wish to accomplish is that if the customer adds the same item to the shopping cart twice the system adds one to the items amount in the shopping cart instead of adding a new line in the cart. Is there a simple code I can add to this code to accomplish this?

Dim row As GridViewRow
        Dim isbn As String
        Dim title As String
        Dim amount As Integer
        Dim price As Double
        Dim t As TextBox
        Dim p As Label
        Dim totalprice As Double

        row = GridView1.SelectedRow

        t = CType(row.Cells(8).Controls(1), TextBox)
        amount = CInt(t.Text)

        p = CType(row.Cells(3).Controls(1), Label)
        price = CDbl(p.Text) * amount

        isbn = row.Cells(0).Text
        tittel = row.Cells(1).Text

        If amount = 0 Then
            Server.Transfer(isbn & ".aspx")
        Else
            Dim shoppingcart As New Data.DataTable
            shoppingcart = CType(Session("cart"), Data.DataTable)
            Dim nyrow As Data.DataRow
            nyrow = handlekurv.NewRow
            nyrow(0) = isbn
            nyrow(1) = title
            nyrow(2) = amount
            nyrow(3) = price
            shoppingcart.Rows.Add(nyrow)

            Session("cart") = shoppingcart
            'regner ut totalprisen i handlekurva og skriver denne ut på siden
            totalprice = Session.Item("Totalprice")
            totalprice = totalprice + price
            Session.Add("Totalprice", totalprice)
        End If