i don't want to add same product to shoppingcart

I wrote this code for adding a product to shopping cart .when user select a product the product add to a list (shopping cart) I want to when user select that product again see a message in label because he selected that product before.

code for adding item to shopping cart
protected void order(object source, DataListCommandEventArgs e)
    {


            if (Session["user"] != null)
        {
            //adding to basket
             if (e.CommandName.CompareTo("Porder")==0)
            {
                string name = Session["user"].ToString ...