I've read a lot of posts and comments on this forum in the past few days but I saw few posts or replies saying "this have been talked about many times before " ,as a fresh forum user like me how will I know if something like that have been discussed before?
That's because many newbies don't even bother to read sticky post and rather make a new topic, and it goes on for years. But if you are looking for something specific, you can try using
https://ninjastic.space/ which makes looking for topics or even keywords much easier.
Also is it wrong for someone else to discuss what a person have talked about before?
It depends on what you want to discuss. For example, it won't be a problem if you have a specific problem and you open a new topic about it (but even then its good to browse forum first to see if someone else had the same problem), but if you come to this section with yet another "Newbies beware of xyz" that's been mentioned countless times before then you might get that response.