onparsedocument output problem

in a plugin when use OnParseDocument the outout repeat ten times,
the code i use is this

$e = $modx->Event;
switch ($e->name) {
case "OnParseDocument":
$a =$modx->insideManager();
if ($_SESSION[’mgrInternalKey’]!=999999999999) {
$html="hello";
$content = $modx->documentOutput ;
$modx->documentOutput = $content.$html;
}else{}
break;
default :
return; // stop here - this is very important.
break;
}

Is it that "hello" is repeated ten times?

This is probably because your plugin is executed 10 times. 10 is also the maximum amount of parser passes. To verify this theory try changing the $modx->maxParserPasses at the bottom of your index.php file and see if the amount changes accordingly.