Why do some pages repeat a previous page's action(s) after redirect?

Hello, I have a form page and a processing page. After submitting the form the processing page does whatever it needs to do (insert a record, send back validation errors, etc.) After determing what to do it always redirects somewhere with header('Location: URL'); But sometimes when I'm back at the form page (after the redirect) and I refresh the page it does the previous page's actions again. And again and again. Why would it do that? Shouldn't a refresh just resubmit whatever is in the address bar and not go through a certain path? The only way I've found to make it stop redoing the previous page's actions is to put my cursor in the address bar and press enter. I don't remember seeing this behavior in the past so I wonder if it has something to do with Apache's or PHP's configuration. Thanks, Chris.