The same page is served on both www and the bare domain

What this means

Within one scheme, the www host and the bare domain both return 200 on their own address and serve a byte-identical document. To a search engine these are two different sites duplicating each other, so every inbound link and ranking signal is divided between the hostnames rather than consolidated on one. As with the scheme check, the two bodies are compared by hashing the first 200KB, and unequal hashes produce nothing. A host that redirects to the other form is not counted, because the 200 then belongs to the destination rather than to the address that was requested.

How to fix it

Choose one hostname as canonical and 301-redirect the other to it in a single hop, then make internal links, the sitemap and rel=canonical all use the chosen host.

Fix effort

This check needs a developer fix: it turns on a routing, hosting or infrastructure decision that no automatic edit can make safely.

Authoritative source

https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls