Hi,
I was rather thinking you would add the URL of an icon image in the "Icon path" setting, under the "HikaShop options" tab of the module.
This would prevent the double display of the fontawesome icon.
Now, this double display can be fixed by adding some custom CSS code, so it's not difficult to do.
However, if you don't want the icon to display when there is something in the cart, then neither of these solutions will work for you. You'll have instead to use a translation override of the CART_EMPTY translation key. In it, you can add your HTML for the fontawesome icon instead of the "cart is empty" message:
www.hikashop.com/download/languages.html#modify