That message appears when counts maintined by the Scoop? software indicate that the message may already have been posted.  I believe the intent is to reduce the instances of posting one's comment twice.  For some reason in some circumstances the software has difficulty distinguishing between "preview" having been pressed and "post" having been pressed.  I usually minimize the screen I am on, open another window of ET and check if my comment was indeed posted.  If not, I go back and press "post."

As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."