Start with the evidence
Google canonical guidance is especially important when extensionless and .html URLs both exist for the same entry.
Before a site adds more language pages, it should decide the preferred URL for each page type and align sitemap, footer links, internal links, and redirects.
Traffic is not the whole answer
The signal matters when it clarifies search intent, proof, and conversion action, not when it adds another traffic tactic.
Check the page promise
- List extensionless, .html, legacy, and language-specific URLs for one page family, then assign one preferred entry
- Keep the test narrow: one priority page with clear topic, source links, internal links, and a conversion action
What still needs proof
Duplicate entries dilute search signals and make readers land on stale copies. Keep the original source open so the announcement, the evidence, and this site's interpretation stay separate.