Duplicate content
Duplicate content refers to identical or very similar blocks accessible at several URLs, within a single site or across sites. Without systematic penalty, it dilutes signals and leaves the engine to select the displayed version — rarely the one intended.
In practice
Common internal sources: parameter variants, print versions, product pages without meaningful differentiation. A canonical tag resolves most internal cases. Across sites, the copier can occasionally outrank the original — engines cross-reference first-indexation dates and authority signals to arbitrate, generally correctly.