People reply with the "already posted" messages so the person will know why their articles was pulled. I've posted a few duplicate articles and I never got upset when someone pointed it out.
I don't mind the Mods pulling a post I've placed up if it's a duplicate. That's a service, actually. What bugs me is the person who points out the redundancy (without a link to the previous thread) as if the rest of us spend 24 hours a day on FR and should have caught it the first time.