Duplicate content = the same (or very similar) content on multiple URLs. Google penalizes pages with internal and external duplicate content.
Duplicate content = the same text (40%+ similarity) appears on 2+ different URLs, internal to your site or on other sites. Types: (1) Internal duplicate: /article and /article/ (with a trailing slash); (2) Similar-content pages (e.g. a short article + the long version); (3) Content distribution: publishing across multiple domains (syndication). Google has to choose which canonical URL to rank — the others are devalued. Penalties: poor ranking, authority split across URLs, crawl budget wasted on duplicates. Fix: use canonical tags, 301 redirects, unique content per URL.
Duplicate content = poor ranking and split traffic. Two pages with the same content rank at 1/10 the value of a single unique URL. 100 unique articles rank much better than 50 unique + 50 re-published.
Source: Google — Duplicate ContentTechnical SEO covers the optimizations of your site's infrastructure — crawlability, indexing, loading speed, HTTPS and data structure — that let Google find and understand your pages.
A canonical URL is the rel="canonical" attribute that tells Google which is the "true" version of a page when duplicate versions exist (e.g. with and without www, with and without a trailing slash).
Hreflang is an HTML tag that tells Google which versions of a page exist in other languages or regions, avoiding duplicate-content penalties for multilingual content.
Google indexing is the process by which Google adds web pages to its database (the Google index) after crawling them, making them eligible to appear in search results.
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