Howto check if alert is up

Im using setInterval to check for data from amfphp.
It checks data correctly but is get a lot of alert windows up if im not clicking Ok.

I want to know how do i check if alert window already is up, and if it is, it wont publish anymore of them.

heres the code im using.

function onNewMess(r:ResultEvent) {
var nya_medd = Alert.show("Det finns totalt: "+r.result+" nya meddelanden", r.result+" Nya", Alert.OK | Alert.CANCEL, this, myClickHandler);
nya_medd._x = 300;
nya_medd._y = 200;

}

I tried with
if (nya_medd.visible = true) but i got undefined and im stuck with ideas how to solve it.