I know what you're talking about, and often find myself hitting the 'ignore' button on certain posts within threads for this reason. These are posts where nothing new is added, and there is no sign of further reflection on reading opposing views since the last 'let me repeat myself' post.
There was a very recent thread where I posted twice, and wondered at the time if I was guilty of what you're describing. Since I added a further thought to elaborate on my view, I let myself off the hook- but the situation is evident enough that I actually asked myself if I was doing the "I know best" dance.
I think posters who do this are passionate and unaware of how it comes across.
ETA- But if my recent double post prompted this thread, please PM me and tell me to knock it off! (And I will