Short definition
The same or very similar content available at more than one URL.
Beginner In plain English
When the same text appears on several pages or addresses, search engines have to guess which one to show.
Professional How pros use it
Usually not a penalty, but it splits signals and wastes crawling. Fix with canonicals, redirects, consistent internal links and unique copy for templated pages.
Examples
- Product pages reachable with and without tracking parameters.
- Location pages with only the city name changed.
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