You want to throttle those users then. There is a setting in the admin under Settings > Writing called “Throttle time”. I think it’s 30 seconds by default. Set that to longer and people will not be able to post again for that long. If you set it to 180, it would be three minutes.
This does not find and remove duplicate posts, but it prevents most of them from happening. The side effect is that if someone is responding to multiple posts quickly, they are prevented from posting to different topics within that throttle time too.
I don’t know of a plugin that will find and eliminate duplicate topics.