Every domain answers at up to four addresses by default: http://example.com, https://example.com, http://www.example.com and https://www.example.com. Unless the server is configured to redirect three of them into the fourth, a search engine can crawl and index all four as separate URLs serving identical content — the textbook definition of duplicate content, created by server configuration rather than anything written on the page.

What the checker does

It requests all four variants of a domain and follows each one’s redirects, then checks whether they all land on the same final URL. A healthy setup has three variants each issuing a 301 redirect to the fourth, canonical one.

What each finding means

• A variant that doesn’t resolve — times out or errors instead of redirecting or answering. If it’s ever linked to from anywhere, that link goes nowhere.
• Variants landing on different hosts — the most serious finding. It means, for instance, the www and non-www versions are genuinely two different sites (or one is misconfigured), not variants of one canonical domain.
• More than one variant answering 200 directly — no redirect at all, just the page itself served at more than one address. Each is then independently crawlable and indexable as its own URL competing with the others.

How this differs from the Canonical Tag Checker

The Canonical Tag Checker looks at the <link rel="canonical"> tag declared in a page’s HTML — a signal that can be present or absent regardless of server configuration. This tool checks the server-level redirect behavior itself, which is the more fundamental fix: a correct redirect means only one URL is ever served as a 200 in the first place, and every crawler arrives at the canonical version before it even reads the page.

How to fix it

The fix lives in the web server or hosting platform’s configuration, not in application code: an Nginx or Apache rewrite rule, a redirect rule in the hosting platform’s dashboard (Vercel, Netlify, Cloudflare all support this), or an .htaccess rule on shared hosting — each 301-redirecting the three non-canonical variants to the one chosen as canonical.

Frequently asked

Which variant should be the canonical one?

https:// is the standard choice today — HTTP is deprecated for anything public-facing. Between www and non-www, either is fine as long as it’s picked once and used everywhere consistently (internal links, the sitemap, marketing materials).

Does this affect ranking directly?

Google’s systems are generally good at picking one variant as canonical on their own even without a redirect — but “generally good at guessing” is worse than removing the ambiguity outright, and ranking signals (links, social shares) can end up split across variants that never get consolidated.