Is this a duplicate content problem?

I have done a redirect on my website, after i have change the url from:https://www.example.com/i-have-done-a-redirect to https://www.example.com/redirect-done Then I use the term site:https://www.example.com/redirect-done and i found out the 2 links have been index on google. I want to ask if this is a duplicate content problem? and if it is a duplicate content, is it the best option to use the remove url function in my google webmaster console or can i just use the robot.txt file to disallow the old url? Please i need help