Actually, I tried with firefox.
Mmm, I tested a bit more and found that sometimes the product isn't added to the cart.
I think it comes for the way the javascript is done: the ajax request which updates the cart is aborted by the browser before it finishes in order to redirect the page. It probably only occurs on your website because it seems a bit slow to load.
Please replace the file administrator/components/com_hikashop/helpers/cart.php by the file in the package enclosed in this post.
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