Either explicitly delete it or set its expiration date to 0 and it will be deleted when the browser is closed.
Personally, I am against opening too many multiple windows but that is a matter of personal choice. I feel that using a cookie or storing the values at the server will serve the same purposes without being as intrusive to the user.
You would have to write that data to a cookie and update it using the onBlur event. WHen the page loads you would have to check for such a cookie and preload all the fields.
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