Just wandering if when someone makes a new thread, it comes up with a list of active threads that are about the same thing..

Possibly if they have the same kind of words in them, that it could come up as ' Is this what you are looking for' or something :P

Because people make the same threads over and over, which gets no where but blocks the space up for threads with a point

The search function was made for this cause but seems to be often forgotten about.

A side bar or something with the last 10 commented threads could be handy or something, but that's slightly different to what your getting at i think.

Seems like it would be a lot of coding and effort for something that would;
1. Clutter up the simplicity of creating a new thread.
2. Make the current search function pointless and unused.

If people are unsure of whether or not their thread already exists, then it's down to them to find out, via the search bar.
If they create the thread anyway without knowing if a similar one exists, it's just a case of an admin taking the appropriate action, whether that be to close the thread, and leave a link to an already active thread with the same discussion, or to link to an inactive thread where the original poster can see what previous thoughts on the suggestion/query were.

It's a good thought, but it wouldn't be as practical as you might think.