"Just add a canonical tag and dupes are solved."

"Just add a canonical tag and dupes are solved."

Canonical is a hint, not a command. Google overrules it constantly.

Here's the checklist to make your canonical actually stick:

— Self-reference on every indexable page. The canonical should point to itself unless it's a genuine duplicate.
— Use absolute URLs, with the right protocol and trailing-slash convention. https://, www-or-not, pick one and never waver.
— Make sure the canonical target is itself 200 and indexable. Pointing at a redirected or noindexed URL confuses everyone.
— Don't canonical paginated pages to page one. Page 2 isn't a duplicate of page 1; that buries the deeper items.
— Check that internal links, sitemap, and canonical all agree. Mixed signals are why Google ignores your hint and picks its own.

Then verify in Search Console which URL Google actually chose. ("Google-selected canonical" differs from "user-declared canonical" more often than anyone admits. The tag is your vote, not the verdict.)