The "duplicate content penalty" for your own site is a myth: "If you have some content that's accessible under multiple URLs, it's fine; don't fret about it." Google clusters the copies, picks one canonical URL and consolidates signals, though duplicates waste crawling. Copying other sites differs: scraping breaks spam policy, and a "significant volume of valid copyright removal requests" demotes a site's other content.
Hints stack, so keep them consistent: a self-referencing canonical on every page, internal links to the canonical form, absolute URLs, and an HTTP header for non-HTML files:
<link rel="canonical" href="https://example.com/green-dress">
Link: <https://example.com/downloads/white-paper.pdf>; rel="canonical"Never canonicalize with robots.txt or noindex, and don't let JavaScript change the canonical link. Search Console 1 's URL Inspection tool shows the Google-selected canonical, which can differ from yours.