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?
If you want to check if the discussion has been made earlier also then you can search across the forum but don't use forum search as it would come up with messed up results that you will find hard to sort out and be confused so I would suggest you to use direct google search for finding if a topic has been discussed before or not and you can check it in the following manner
Who is satoshi bitcointalk
How to make tables bitcointalk
The FTX crash in the market
This way you can check if the discussion has been made or if you can then explore
Ninjastic.space to check for the new and old threads in the forum.
Also is it wrong for someone else to discuss what a person have talked about before?
No, it's not a problem and there are many threads in which past discussion are also made but the point you are providing or discussing must be relevant but if we are discussing the same old thing again and again then it's not a fun and you will understand it once you see those threads.