Rahul asked Google's John Mueller on Twitter if Google would display an old URL of a site in the Google search results even if it is redirected to a new URL and if so, could that be a duplicate content issue. The answer is yes, Google can show the old URL in the search results and no, it is not a duplicate content issue.
Here is an example of searching for [seoroundtable.com], most people confuse this site with seOroundtable.com vs SEroundtable.com. The SEO version redirects to the SE version as a 301 and has done so for years and years. But if I search specifically for the SEO version, Google will show it:
But if you click on it, Google will first take you to seoroundtable.com and then the browser will follow the redirect to seroundtable.com.
Google's John Mueller said this happens when a searcher specifically searches for the old URL:
And no, it won't be a duplicate content issue, John added:
Forum discussion at Twitter.