Thanks for this contribution it helps a lot but when I try to add additional quantity on the product it will not add in the quantity in shopping cart.
First, I add in the cart and choose the quantity 10 in the drop down quantity box and the quantity is added in the shopping cart.
Second I back to product listing or in product info and try to add additional quantity 20 for the product and the results is that the quantity change to the new quantity I choose, instead the quantity must add the new quantity e.g. 10+20=30 the quantity must be 30 not 20. Is there any way to solve the problem and thanks a lot.

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