Core output, SEO plugin and theme head
Check whether the active theme displays the normal head area and which SEO plugin canonicals manages. Do not add a second snippet unchecked.
On a single content page, check if WordPress Core, SEO plugin, and theme each output canonicals. For archives, a different logic may apply than for contributions. Do not blindly remove the core function, but first determine the leading source.
Where to check the setting in WordPress
Open the head of a published single view and search for rel=canonical. Assign the output to the core call rel_canonical, the active SEO plugin or your own theme snippet. A second manual output in the header does not fix an incorrect plugin value.
Example of this specific finding
Illustrative control case: An article page and its tracking variant provide the same text, but not a preferred URL. The variants must first be compared in terms of content.
<!-- Auf Artikel und identischer Trackingvariante -->
<link rel="canonical" href="https://example.org/ratgeber/bremsen/">follow-up audit in WordPress
Compare a post, a static page and an archive. The additional tag comes from a theme snippet, the change of which is first checked in a test environment.
For these identical variants, consistently specify the clean article URL. Internal links and sitemap should give the same address.
Derogations and scope
A unique page without a canonical is not automatically unindexable. Even an existing canonical does not guarantee the selection by Google.
Related: WordPress with classic or block theme. SEO plugins and page builders are different; there is no universal menu path for all installations.
Classify the general error
Definition, system-independent causes and further correction examples: A missing canonical is not a blanket indexing error