AS3 Random Image from XML - Never Repeat

I'm creating a banner ad in flash which shows 3 random images from a XML file once the pages loads. My problem is that it will occasionally repeat. I've included my code, any advice?:hubba:

var pHeight:Number = 84;
var pWidth:Number = 120;


var listLoader:URLLoader = new URLLoader( new URLRequest("www.example.com/example.xml") );
var picLoader:Loader = new Loader();

listLoader.addEventListener(Event.COMPLETE, gotList);

function gotList(evt:Event):void {
var xmlData:XML = XML(listLoader.data);
var numImages:Number = xmlData.picture.length();
var stImage:String = xmlData.picture[Math.floor(numImages*Math.random())].toString();

picLoader.contentLoaderInfo.addEventListener(Event .COMPLETE, gotPic);
picLoader.load( new URLRequest(stImage) );

listLoader.removeEventListener(Event.COMPLETE, gotList);
}

function gotPic(evt:Event):void {
var thisBmp:Bitmap = Bitmap(evt.target.content);
thisBmp.x = 0;
thisBmp.y = 0;
var thisWidth:Number = thisBmp.width;
var thisHeight:Number = thisBmp.height;

thisBmp.scaleX = pWidth/thisWidth;
thisBmp.scaleY = pHeight/thisHeight;
addChild(thisBmp);

picLoader.contentLoaderInfo.removeEventListener(Ev ent.COMPLETE, gotPic);
}