Joomla has a built-in duplicate content issue. When you enabled built-in Joomla SEF on your site, non-sef urls can still be accessed. For example, you have a SEF url like
https://www.your-good-domain.com/your-good-sef-url.html
it can still be accessed like
https://www.your-good-domain.com/index.php?option=com_content&view=article&id=10
See? with article id anybody can access it and so Google. This can be a bad impact on your site ranking and your rank in SERPs.
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