In more detail
Most duplicate content isn't deliberate. Common causes include:
- The same page reachable with and without
www, with and without a trailing slash, or over both HTTP and HTTPS - Tracking, sorting and filter parameters creating new URLs for the same listing
- Product variants (colour, size) each with their own near-identical page
- Printer-friendly versions, staging sites left indexable, or syndicated articles
When Google finds duplicates, it groups them and picks one URL as the canonical to show in results. Google's guide to consolidating duplicate URLs explains how to tell it which version you prefer, using canonical tags, 301 redirects and consistent internal links.
Duplication is different from deliberately copying content to manipulate rankings. Google's spam policies cover scraped content and scaled content abuse, which are treated as spam. Ordinary technical duplication is a housekeeping problem, not a policy violation.
Duplicates cause practical harm. Links and signals may be split between versions, Google may show a URL you didn't intend, and on large sites crawlers waste time on copies instead of new pages.
Why it matters when hiring an agency
Almost every SEO audit finds some duplication. What matters is whether the agency can tell you how much it matters and the simplest fix. On a small site, a few duplicate URLs may be harmless. On an ecommerce site with thousands of filter combinations, it can be the main technical problem.
Be wary of any agency that describes duplicate content as a "penalty" to create urgency. Ask instead which duplicates affect important pages, and how they will confirm which URL Google has chosen. Search Console's URL Inspection tool shows the Google-selected canonical.