Special thanks to Pam at League of Women in Government for asking this great question: What is a canonical URL?
What if you publish an article that gets replicated on a news site, or what if you have the same product multiple times with slightly different settings in your eCommerce site, or what if you want to syndicate a blog? The answer: you want visitor coming to your site from a search engine to go to a more relevant page. This is the purpose of the canonical URL: to inform the search engine that a more relevant page exists for this content.
Literally, a canonical URL looks like this:
<link rel="canonical" href="http://example.com/a-better-page" />
Adding a Canonical URL
The WordPress SEO by Yoast plugin is a great search engine optimization tool. To add a canonical URL to a page on your site using this plugin, in WordPress edit the page that replicates the original content, scroll to the bottom, and in the Yoast SEO settings, choose advanced (gear icon), and then enter the URL.