Hi,
1. Duplicate URLs for the same page is only a problem if you don't enter a "canonical URL" in your product.
If, for that product "myproductname", you enter the URL /e-boutique/product/myproductname.html in the "canonical URL" field of the product, then search engines will only reference that URL for the product page and won't count the other URLs leading to that page as "duplicate" and thus it won't affect your SEO.
2. Yes, you don't have a unique URL for a page in Joomla. For example, if you configure two menu items of the type "single article" and select the same article for both menu items, then both menu items will display the same article, only the URL will be different.
And for the product, category, or category_pathways parameters of the URLs, it's basically the same as the menu alias in the URL. It allows the router to know which page of which menu item is requested.
Without them, the router would have to do many queries to guess what is called.
For example, for the URL
abc.com/e-boutique/mycategoryname/myproductname.html
, it would have to search and see if myproductname is actually a product alias or a category alias or a brand alias or something else.
Also, If you add the product to the cart and go to the checkout, you're still on the same menu but you want to have something in the URL to know that it's the checkout page you want to display. So you'll want for example
abc.com/e-boutique/mycategoryname/checkout.html
But then, if you have a product with the alias "checkout", then it would match with that URL and you would actually be unable to access the checkout anymore after creating a product with that alias.
In short, making a routing system which works in all the cases that you can setup your shop requires to add some parameters to know "where we are" when routing the URL.
Now, if you really want the URLs like that, it's totally possible. But you need to use a SEF alias system like there is in sh404SEF so that you can tell it for example that for the URL
abc.comt/e-boutique/product/myproductnam...gory_pathway-12.html
you actually want to get the URL
abc.com/e-boutique/myproductname.html
or
abc.com/e-boutique/mycategoryname/myproductname.html
and it will do it as it has a corresponding table of URLs in the database.
3. For the example you provided via our contact page:
You entered the canonical URL /e-boutique/granules-test-canonical/aliment-complet-pour-poule-pondeuse.html in your product.
However, that URL is not a valid URL of that product page. You can't write whatever you want in that canonical URL field and it will work. You need to enter a valid URL of the product page. If you want to have whatever you want for the URL of the product page, you can use sh404SEF as I was saying earlier. Otherwise, you have to have a valid URL as per the structure of the HikaShop URLs and the way you setup your shop.
Since you have the "product SEF name" set to "product", a valid canonical URL of that product would be instead:
/e-boutique/granules-test-canonical/product/aliment-complet-pour-poule-pondeuse.html
However, if you want to keep the canonical URL /e-boutique/granules-test-canonical/aliment-complet-pour-poule-pondeuse.html, it's possible, but in that case, you want to empty the setting "Product SEF name" in the HikaShop configuration.