Hi
How do we make sure that there is only one category page in the indexing:
These pages are not the same, they do not constitue duplicate content. They each have a canonical to self, which is usually not needed because, again, they are not the same pages.
For instance:
https://solt74.ru/manufacturer/euroboor/ is a page to all products from manufacturer euroboor
https://solt74.ru/manufacturer/euroboor/masla is a page to all products from manufacturer euroboor of type "Масла"
So in essence these are subresults. Personally, I would no index all these pages, the /xxx/masla ones, as they are just refined search of the manufacturer pages.
It has been argued that canonical tags should be used on all pages to avoid any unintended search engine duplication issues, whether you duplicating your content, or not
Yes, but only if the correct canonical are in use. And that's usually really hard to do.
To fix duplicate link errors, you can use the plugin Canonical Links
An example of what I was saying, where this kind of plugin insert incorrect canonicals in mosts cases, as they just create a self-canonical.
Best regards