"Already existent topics" feature
I'm sorry but there is not an easy way to do this. Also, this will have an impact on the website performance.
In case you want to say thank you !)
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It would have an impact on the performance, as the "Topic Name" field would function like an Ajax search field. The impact would however be restricted to the moment of the creation of the topic. Then this option should have a checkbox to activate/deactivate it, for every need and taste.
This feature as well as “Similar, related topics” feature under every topic are present in most of the dedicated forum apps (outside Wordpress).