Hi,
From what I can see, the product added is the one in the module "Products you might like" that you have on the right side of the product page.
Looking further into it, it seems to be linked with the javascript code of the add to cart button not working properly, maybe due to the jchoptimize extension you have which modifies the javascript code of HikaShop.
Try to deactivate it and see if that helps.
Otherwise, removing the add to cart button on the module on the right side would also be an easy alternative.