You put up a registration page on your site, hoping that visitors will leave you their email addresses so that you can stay in touch with them when you've got a new product for sale. Or a new tutorial that they might be interested in. Or when you want to send them some "information from carefully screened third parties with whom we maintain a strategic relationship." Or maybe you want something in return before you give them that valuable whitepaper that you spent two months on.

Whatever the reason, you happily construct your registration page, set up a database table to track the incoming email addresses, and push it live. And sure enough, the registrations start coming.

To mickey@mouse.com. And donald@duck.com. And emailthis@hahaha.com. You get the idea - users are registering with bogus email addresses at domains that don't even exist. Not only are you going to be sending mail to nonexistent addresses, but they clutter up your database and cause maintenance headaches because they need to be cleaned out.

After a few moments of discussion, her problem became clear: she had an application where a link on one web page would open up another browser window that contained a series of forms for the user to fill out. The problem was that if the user mistakenly returned to the first window and clicked the link again, then the second window would be refreshed and all of the work the user had done up until that point would be lost. "Is there a way," she asked, "to prevent the second window from refreshing?"

Well, there's actually an even better solution than that one, which will be the subject of this tutorial.