Navigate away message appears twice -

I want to warn users if they try to leave a page without saving changes. So, I have this on the page:

<script for="window" event="onbeforeunload" type="text/javascript">
if (PageEdited)
{
event.returnValue="You have altered some of the data on this page but you have not saved your changes.";
}
</script>

I have a global variable - PageEdited - which is set to true if a user types in a text box or changes a dropdown list etc.

I have used this code on lots of .asp sites with no problems. But on my first .net site - the code fires twice. i.e. The alert that says 'Are you sure you want to navigate away from this page' fires twice - if I click on a hyperlink.

But, if I click on the close button (the browser close button at top right of window) - the alert only appears once.

Any ideas why the alert appears twice when I click on a hyperlink?

Posting this here because I think it might have something to do with 'postbacks'.

Thanks for any help.