Duplicate Content
Substantially identical or very similar content appearing at more than one URL, whether within a single site or across domains. Search engines then must choose which version to index and rank, which can dilute signals and leave the wrong version surfacing, or none ranking well.
Why It Matters
Duplicate content splits link equity and ranking signals across copies, weakening all of them. Resolving it consolidates authority onto one canonical URL and ensures the version you want is the one that ranks.
Common Mistake
Letting parameters, print versions, HTTP and HTTPS, or www and non-www serve the same page at multiple URLs without canonical tags or redirects. Copying manufacturer product descriptions verbatim is another common source.