Duplicate content
The same or very similar content available at more than one URL, on one site or across several.
Duplicate content is normally not penalised: search engines pick one version to show and fold the others into it. The cost is choosing the wrong version, and splitting links and crawling between copies. Canonical tags, redirects and consistent internal links steer which version wins.
Copying other sites' content wholesale is different: that is scraped content, covered by the spam policies.
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