Like kanga, I too have had problems with the preview function. I think you missed the drift of what kanga said... after discovering a mistake during the preview function, we go back and "make a change" to the post then attempt to preview again or post and get the Duplicate Submission message. This tends to negate the advantages of the Preview function since in effect it prevents you from posting at all. I've pretty much given up on preview and just post. So, like kanga I trust everyone will forgive the typos and gramatical errors that creep in... it's too dificult to correct them. I've seen this problem on other sites using this software so don't feel alone!
PEACE-THE FINAL FRONTIER!
Uncle Bob