Are you sure about that? Once you've instructed the browser to navigate elsewhere it shouldn't still attempt to retrieve some other web page. That should be a trip into onunload and then to the new page with no excursion for the other page which would have been loaded otherwise.

I'd consider the browser that requested the page twice to be incorrect and curse it a bit. I'd still fix the page so it worked correctly regardless but be sure to harbor some ill will to the poorly performing browser.