It is quite common to see users making duplicates of topics because they originally posted them in the wrong category. Sometimes after a user posts a topic in the wrong category and someone else tells them to change the category what will happen is that the original poster will just copy and paste the text and create a new topic in the category that got suggested.
Because users don’t bother learning about the forum, suggesting the author to use the edit button on the initial topic instead of making a new one if this one has the same title would be a good solution to avoid this kind of duplication of topics.