Canonical tags are the primary technical solution for duplicate content — one of the most common issues found in SEO audits. When search engines encounter multiple URLs with identical or very similar content, canonical tags tell them which version to index and rank.
Why Duplicate Content Happens
E-commerce filter URLs, URL parameters (tracking codes: ?utm_source=), www vs non-www versions, HTTP vs HTTPS, trailing slash vs no trailing slash, paginated pages, and content syndicated across multiple domains all create duplicate content issues.
Canonical Tag Implementation
Add <link rel="canonical" href="https://domain.com/preferred-url/"> in the <head> section of each page. Self-referencing canonicals (a page pointing canonical to itself) are best practice for all pages — prevents Google from making its own canonical selection.
Canonical vs 301 Redirect
Use 301 redirect when you are permanently retiring an old URL. Use canonical tag when the duplicate URL must remain accessible (e.g., e-commerce filter pages that users navigate to). Canonicals consolidate SEO signals without changing user-facing URLs.
Common Canonical Mistakes
Canonicalizing to a noindex page (consolidates signals then removes them), canonicalizing to a redirect (creates signal chain confusion), self-contradicting canonicals (Page A canonicals to B while B canonicals to A), and forgetting canonicals on paginated pages.