GSC shows "Duplicate pages, Google has selected a different canonical page." What's the most reliable way to handle this?

This is Google's duplicate content guidance, not a penalty. Even with canonical tags set, Google will still determine the canonical page based on signals like internal links, sitemaps, and URL structure. Common causes include crawlable parameter/pagination pages and inconsistent URL signals. As long as Google selects the canonical page you intended and core pages are properly indexed, no action is usually needed.