Adding items to cart which are already there.

I have a simple web form with a button on it.  When it is clicked it will had an item to a
shopping cart.  What I want to achieve is
- if the same item is already in the cart the quantity is simply increment by
1. 





So far the item goes to the cart no problems with and an
order number associated with a session ID)






Can anyone point me in the right direction?
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