You cannot send headers once you have sent ANY html to the browser…so if you want to do whatever it is you’re trying to do with the headers, you’ll need it to be before get_header().
On that note, I’m not sure what you’re trying to do with the headers, but the ‘Confirm Data Resubmission’ prompt is coming from your browser. Do a fresh load of the page and then refresh and you shouldn’t see it…but there’s no way to get around it (because, again, it’s from the browser)