Add sorting and filtering to a listing page and you get addresses ending in ?sort=new. On screen it is the same page in a different arrangement; to a search engine, a different address is a different page. Combine a few options and you have dozens or hundreds of URLs carrying nearly identical content.
What makes this awkward is that nothing looks wrong. The site works, visitors are happy, and no error is logged anywhere. It happens quietly, and only on the search side.
One page, many faces
Two costs follow. First, the time and requests a search engine spends on your site get spread across the copies, and the genuinely new page waits its turn. Second, you no longer choose which version appears — a URL with a sort parameter really can end up as the one shown in results.
Canonical — “count this address”
A canonical tag is one line inside the page saying “the primary address for this content is here”. When the copies all point at the primary, the search engine has a basis for picking one to index.
It is a strong hint rather than a command, though. If the address you nominate does not match the content, it can be disregarded. Using it well therefore comes with conditions.
Pagination is where this goes wrong most often. Make page two point at page one and everything that only exists on page two disappears from the engine’s view. Each page of a listing should name itself.
What a marketer can check, and how to ask
With one SEO plugin active, WordPress gives every page a self-referencing canonical by default. So the problem usually arrives not with the base structure but with whatever was bolted on later — listing filters, on-site search results, shared links carrying campaign tracking parameters.
Check in two places. In the Search Console pages report, watch whether duplication-related reasons are growing; then run a site: search and see whether parameter-laden URLs appear. If either shows up, write the request like this: “These parameter URLs are being indexed. Please make them all point to the parameter-free address as canonical.” Target, observation and desired state in one sentence is what turns a comment into work.
Duplication seen from the content-design side continues in the SEO archive, and clearing up duplicate addresses is part of the technical review in our optimization program.
Next part
With addresses settled, it is time to describe the page to machines. Next: structured data — where the real decision is choosing what you actually are.