How to search an Array class for duplicate entries

 I have a Cart program that works, except that if you add the same item it appears in the GridView as a second entry. I would like to check the array class and search through it to see if the product id already exists before creating a new entry in the gridview. If it is there then I will simply change the amount and extended cost. If it does not exist then it is a new entry and should be added. Here is what I have but it is not working. Can someone help???
 
 
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using Sys ...