Canonical URL is set on each page

The same page reached via /about, /about/, /about?ref=twitter, and https://www.yoursite.com/about is four separate URLs to Google, and PageRank splits across all of them. Duplicate indexing dilutes ranking signals, triggers canonical-selection on Google's side (often picking a URL you did not intend), and on e-commerce sites causes faceted URLs to consume crawl budget that should go to product pages. A canonical tag consolidates all signals to the version you choose.

Set alternates.canonical per page in Next.js metadata, pointing to the absolute URL you want indexed. For dynamic routes, use generateMetadata so each slug gets its own canonical.

// app/[slug]/page.tsx
export async function generateMetadata({ params }) {
  return {
    alternates: { canonical: `https://yoursite.com/${params.slug}` },
  }
}

For the Pages Router, emit a <link rel="canonical" href="..."> inside next/head. Always use the HTTPS, www-or-not variant that matches your production hostname.