It looks like a link element with rel canonical in the head of the page, pointing at the URL you want indexed. Duplicate addresses appear constantly without anyone intending it: with and without www, http and https, trailing slash variants, URLs with tracking parameters, printer friendly versions, and category paths that surface the same item twice. Left alone, search engines pick a version themselves, which can split signals or index the wrong one. A canonical consolidates them. Google treats it as a strong hint rather than a command, so it can still choose differently.
Most service sites need very little attention here, but the mistakes are costly when they happen. A site wide canonical pointing every page at the homepage will remove the rest of your pages from the index, and that misconfiguration shows up regularly after a theme change or a careless plugin setting. Self referencing canonicals on each page are a safe default. If you syndicate a blog post to another site, ask them to canonical back to your original. Check the tag on a few pages after any site rebuild.
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