whmcs - buy 1 quantity but invoice shows 2 quantity

i created a multisite and whmcs mu. it was configured correctly. from signup until creating the multisite. then i duplicate the product in the product/service section in whmcs. after that whenever a client want to buy that product, it will times 2 on the quantity. when add into the cart, it is 1 quantity. after click complete order, then 2 quantity shown in the invoice. i tried delete the duplicate product, delete all product, replace all the whmcs mu provisioning files with stock files but none work. when the order placed, it placed as 2 identical item.