Duplication of content is one of the most penalizing principles to be able to rank on a search engine like Google. We speak of duplicate content when pages, belonging to the same domain or spread over several domains, have identical or substantially similar content.
This problem may also come from a site that does not correctly handle extensions. Example:
https://www.mysite.fr/my-page.html
Choose a single version of page extension for the entire website (preferably without ".html", ".php" ...). Enter the url in one version and apply a 301 redirect on the other version that redirects to the preferred version.
Prefer no extension in the URL to make the URL as intelligible and easy to read as possible.
Example of directive to place in your .htaccess file to redirect all URLs including a ".html" extension to the URLs without the ".html" extension.