In case you use Magento Blog extension and face the issues of double canonical tags on blog pages know that it most commonly happens because you use some third-party SEO extension. And that SEO extension adds the additional canonical URL for your blog pages.
If the SEO module you use is Mageworx SEO extension you need to go to Stores > Configuration > Mageworx > SEO Base > Canonical URL Settings and set the following items in the Canonical URL won't be added for those pages:
Once you add them, the second canonical URLs won't be set for blog pages and the native blog canonical URLs will be used.
In case you use SEO extension provided by some other company we recommend you to contact them and ask to provide documentation or guidance on how to remove the canonical URLs set by their extension from Magento 2 blog pages.
We also recommend you to use only native Magento 2 blog canonical meta tags. From our experience, SEO extensions do not usually build correct canonical URLs.