If Google chooses a different canonical URL than the one you specified, start by using the URL Inspection tool to see which page Google considers canonical. Evaluate whether Google's choice better serves users.
Next, check for technical misconfigurations such as incorrect rel="canonical" tags, server errors, or missing hreflang annotations, especially if you serve similar content to different regions. Ensure that pages clustered together as duplicates have clear and significant content differences to help Google distinguish them.
After making fixes, request re-indexing via Search Console for your most important URLs, but note that re-evaluation can take up to two weeks. Regularly verify your CMS and server settings to prevent unexpected canonical signals. [Google]