How to update row in entity framework

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C#

database

ListBox

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i am making a pos software and now my clients request is when they add 1 items 2 times in listbox so it's just show one time but quantity of this item increase i am working on this but i dont know how to work on quantity.

Button b = (Button)sender;
tblProduct tp = (tblProduct)b.Tag;
string product = tp.productName;
lbProductsToBuy.Text = tp.ToString();
products.Add(tp);
total += (decimal)tp.productPrice;
var duplicates = products.GroupBy(i => i.productName)
                 .Where(x => x.Count() > 0)
                 .Select(val => val.Key);

 foreach (var item in duplicates)
 {

 }
and listbox format is

   try
            {
                string ProductName = ((tblProduct)e.ListItem).productName;
                string Price = "Rs: " + String.Format("{0:}", ((tblProduct)e.ListItem).productPrice);
                string ProductNamePadded = ProductName.PadRight(33);
                e.Value = ProductNamePadded+Price;
            }
            catch
            {
            }

look at this image of issue
\http://imgur.com/a/1DhiV

What I have tried:

0
down vote
favorite
i am making a pos software and now my clients request is when they add 1 items 2 times in listbox so it's just show one time but quantity of this item increase i am working on this but i dont know how to work on quantity.

Button b = (Button)sender;
tblProduct tp = (tblProduct)b.Tag;
string product = tp.productName;
lbProductsToBuy.Text = tp.ToString();
products.Add(tp);
total += (decimal)tp.productPrice;
var duplicates = products.GroupBy(i => i.productName)
                 .Where(x => x.Count() > 0)
                 .Select(val => val.Key);

 foreach (var item in duplicates)
 {

 }
and listbox format is

   try
            {
                string ProductName = ((tblProduct)e.ListItem).productName;
                string Price = "Rs: " + String.Format("{0:}", ((tblProduct)e.ListItem).productPrice);
                string ProductNamePadded = ProductName.PadRight(33);
                e.Value = ProductNamePadded+Price;
            }
            catch
            {
            }