Hi,
Your product page have some HTML errors, it result a broken form and when you submit the page the element "product_id" is sent two times.
You can use your browser console to see the data sent by your browser when you add the product in the cart.
I recommend you to check your view overrides.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.