There are way too many repeated topics on the forums. There must be literally 500 3D Scratch topics. Maybe, when you are typing the topic title, suggestions underneath should show of other topics with a related title. If you'd click on the link it'd take you to that topic. So if you're typing in the word “3D” into the topic's subject, a scrolling list of 3D Topics would appear underneath, to let you know there is already a topic on it.

This is a nice idea, but it would be difficult to program it to catch more than just a few keywords.
I also foresee a problem where if it triggers a false positive, a user will not post their topic, even if it is real and the filter just made a mistake, and we may miss out on an important suggestion or issue.

I support this idea.

If you want to see a web site where they have this functionality, just check out http://www.stackoverflow.com When you enter a new question, after typing the subject line you are given a list of possible duplicate questions. It works really well.

If this would be just like the Scratch Suggestions page, that shows existent suggestions based on your keywords, then I support.

But I'm not sure how they're going add that function to these Forums…

Bump. I believe this is still relevant.

Support, but I assume it would be very complex to implement.