If the same article appears on two different URLs on your site, Google doesn't know which one to show in search results. This is duplicate content, and it dilutes your SEO power. It happens in two ways: someone copies your content, or you accidentally publish the same text on multiple pages.
Google doesn't penalize duplicate content outright, but it won't rank it well either. The search engine sees it as unoriginal and may choose to display neither page. The fix? Always create unique content, and use canonical tags when you genuinely need the same content on multiple URLs to tell Google which version is the original.