the notice will only be once unless you are viewing a cached page in your browser...if you use the back button to go back to the original page that the pm loaded on and it has the popup code cached in it then it will continue to popup...if you clear your browser cache this should fix it as well so you get only one notice...also the PM link at the top menu will turn red when there are messages and say "New PM's" instead of the regular color "PM" link.