Hi,
Ideally, you want to :
1. edit each product in your backend, and fill the "canonical URL" field with the main URL you want your users to use to access the product.
2. set the "force canonical URL on listings" setting to "use canonical if specified".
An alternative is to set the "force canonical URL on listings" setting to "use canonical URL and generate if missing". With this, HikaShop will automatically fill the canonical URL of the products. The first time a product is listed on a listing of the frontend and that product doesn't have a canonical URL, it will use the currently generated URL as canonical URL.