I know i should probably know this but when it comes to reporting posts.

Should you report everything that you see that goes against site rules(within reason) or just posts that are seriously in breach and could cause hassle

for example, lets say somone posts porn, or the pear or something, obviously this warrants a report, as it is actually quite troublesome.

But when it comes to something like in the Bands and Artists forum and all its subforums where someone mistakenly creates a duplicate thread for a band. Obviosuly you post and tell them a thread already exists, but do you report it as well?

I would, but i just wanna be sure
Yeah go ahead and report it. Then a mod can come close the thread. It's unlikely that the poster will get warned for a very minor infraction like that. So you aren't hurting anyone. Just helping to keep the forums less cluttered.