Hi,
HikaMarket is using the HikaShop import helper in order to copy a product ; just like HikaShop is doing in the backend for product copies.
$importHelper = hikamarket::get('shop.helper.import');
if(!$importHelper->copyProduct($product_id)) {
So if you have an issue with the copy in the front-end ; you might have the exact same issue in the backend.
And in that case ; best is to create a new topic in the HikaShop forum section for the issue in the HikaShop import helper.
Regards,
PS : HikaShop 2.6.6 is not a valid version number.