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?
Use Google or Bitcointalk's search function to check if a certain topic has already been discussed or not.
Also is it wrong for someone else to discuss what a person have talked about before?
It depends on what the topic is some topics have been discussed dozens of times without any new information/narratives(who is Satoshi, etc), while some topics are fine to be discussed redundantly due to new information(exchange reviews, etc).