Confirm the two pages really are equivalents
If the declared canonical and Google-selected URL answer different intents, the tag may be inappropriate. Canonicalisation works best for identical or substantially similar pages.
Map every canonical signal
Compare redirects, HTML canonicals, HTTP headers, internal links, sitemaps, hreflang, protocol, hostname, trailing slash, and rendered content. Mixed signals make the preferred representative unclear.
Choose one destination and reinforce it
Link internally to the preferred URL, list it in the sitemap, output one direct canonical, and redirect duplicates that should disappear. Avoid canonical chains and destinations that return errors or noindex.
Allow time for reprocessing
After material fixes, Google must recrawl the affected cluster. Read the complete canonical guide before making template-wide changes.