Stock Control Programming Help!

i'm currently creating a Stock Control Program and i got to the stage where i created a form that checks the stock by using a barcode scanner ,and the code is scanned into a text box and the code is checked against the database to see if the product exist and if so, then it adds the product name into a listbox.

But i don't know how to make it count the number of similar items in the listbox and how to carry that out with different products when checking stock!

Pls give any suggestions would be appreciated!