Can I use canonical URL on the same page it links to?

I want to ask about canonical URL. Lets assume this URL structure:

/category_A/page1
/category_B/page1
/category_C/page1

Simply put, there are multiple URLs that display the same content. But only /category_A/page1 is original(canonical) page. My question is if it have any impact(bad/good/neutral) considering SEO if I will have canonical URL pointing to /category_A/page1 placed in this same page /category_A/page1 ?

The reason for this is, that its much easier to just put canonical URL into head of every page than to trying find out in which page开发者_运维百科 it is not needed (its the same page as canonical one)

---

just do it

you described the perfect use case for the canonical tag. the canonical tag is a kind of failsafe against wrong URLs (or URLs with unnecessary parameters). so it's good to have it even on the page with the URL the canonical points to.