I've had some similar experiences like what you describe. Whenever I felt a post was too close to mine I just left it up to the instructor to figure out if I was ripped off or not. I never actually contacted the instructor with something like that but instead I would participate in the other posters thread and point it back to my own reply, it's a subtle but effective way of saying "hey, I already posted this". I encourage you to contact the instructor if you feel like your work is being copied or you are being harassed in class, there is usually a zero tolerance policy on such things.
That being said I’d like to share my feelings on the matter.
First, what I experienced was a debate which at times would become heated. I personally love a heated debate as it is a symptom that you have someone who is ready to contend with you about something they have passionate feelings on. It is so much better than the “me too” replies as this will challenge you to dig deep, gather facts, support your opinion and poke holes in their argument. It will make you so much stronger than a “me too” post will and when you get done with the debate, you will have learned some new.
Maybe your opinions will change as a result of the debate, maybe not, maybe theirs will, maybe not. Either way you will take something away from that experience that cannot be substituted by any other method in my opinion, you will be support your beliefs and opinions using cited sources, critical thinking and developed communication skills. Sometimes people may yell which isn’t appropriate but that’s cool too, that usually means you’ve got a point 
so take it in good cheer. As long as it isn’t a personal attack I say let them vent all they want. You simply get an axe of truth and cut away at their argument until either winning, losing or reaching a stalemate.