Check for matching string and update datagrid row

Group,
I am working on a quote form which basically has the same functionality as "Add to Cart".  What I am doing is entering a qty, looking up a stock code and hitting the "add" button.  It will then add the qty, stock code, etc. to a data grid.  The issue I am having is if I enter the same stock code and click add it adds a new line instead of just updating the line that already exists.
Kind of hard to illustrate, but and example would be entering 1(qty) ABC(stock code) and add.  then enter it again and click add I show:
1    ABC
1    ABC
instead of
2    ABC
Not sure what I am missing in my code, but below is what I for that part.  If it doesn't match I tell it to write create a new datarow.
objDT = (DataTable)Session["Quote"];
object Product = txtProductSearch.Text;
//This s ...