I would have to agree with you mostly. Even though I've been guilty of it myself. Alert the mods, let them decide what stays, and what goes.
On the other hand, I personally like to be told if I've made a duplicate thread. As long as I'm given a link. That way, I can go check it out, and
join in.
I think if a moderator notices it and decides not to take any action then people should butt out and mind their own I understand people don't like
it but 1) There's no need to be snide about it and 2) If the moderators don't care why should anyone else.