It is tricky because on the one hand, you have to do something to ensure that people searched before creating a new, duplicate topic. And unfortunately, relevance through searches is a super hard problem.
Contrary to @sam’s observations I have had positive results when creating a new topic and getting a duplicate topic search hit from the “your topic is similar to” panel. Others have reported positive results on Twitter as well if you browse the @discourse Twitter account.
My problem is I rarely create new topics in the first place, and on the rare occasion that I do, I know they are guaranteed to be new topics, not duplicates. Probably the same for Sam. To be honest this is a feature for your, uh, “less experienced” users.