We've been seeing a lot of duplicate threads, and this is because people have either not read the rules and/or haven't made double sure a thread has already been made before making one about the same thing.
There is a search feature next to the NEW TOPIC button. Go to that and search for the thread you're wanting to make. If you see a thread already made, don't make a new thread. If the thread is old, it's okay to bump it as long as what you post is meaningful, relevant, and contributes to the topic.
If you're new and haven't looked at the forum rules and rules for all the subforums you're going to be posting on, then do it. The forum rules are a global announcement, meaning they show up in every subforum, and rules for subforums are stickied, meaning they are at the top of the subforum they're located in.
Please make double sure that a thread of the same thing hasn't been made before you make a new thread.
EDIT: If you want to narrow down results if the quick search feature is showing up too many posts, then use this: http://www.supermariobrosx.org/forums/search.php
Type in words of the thread you want to make where it says Search for keywords. Whatever subforum you're making the thread in, select it where it shows a list of all the subforums. Where it says Search within, select Topic titles only. When it shows results, look for the thread.
Quick search works but if you want to narrow down results even further, use advanced search.