Sitemap lists a non-canonical URL
What this means
The listed URL names a different address as its canonical. A sitemap should carry the version you want indexed, so listing the duplicate sends Google to the copy rather than the original.
How to fix it
List the canonical URL instead. Where a URL is canonicalised elsewhere, it usually should not appear in the sitemap at all.
Fix effort
This check needs review: the change can be drafted for you, but a person has to approve the wording or choose the target before it ships.
Authoritative source
https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls
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