By using a META tag, you can set the page to expire on a date that has already passed, and use the Pragma parameter to tell the browser not to cache the file. This will prevent the user from hitting the Back button on the browser and resubmitting the page multiple times. Put this code into the $$HTMLHead field:
"<META HTTP-EQUIV=\"expires\" content=\"Wednesday, 27-Dec-95 05:29:10 GMT\"><META HTTP-EQUIV=\"Pragma\" CONTENT=\"no-cache\">"
Note that this does not work w/ MSIE. MSIE needs an HTTP header to be set.