how to prevent resend data after press refresh button

I have main login page (index.php) which have 2 field user id and password, after key in User ID and Password, i will register the user id as as session variable, then page will redirect to process.php, and in this page i have a logout button which will redirect to logout.php and do: session_unset(); and session_destroy();. After logout, i right click on back and click on index.php the page expired message appear and i press refresh button and the alert came out and ask for resend data and i press yes to resend data, the page will directly goto process.php and not remain on index.php, how can i make it stay on index.php, please help --------------------------------- Do you Yahoo!? Free Pop-Up Blocker - Get it now