When you use the browser back and forward buttons it relaods from the cache (local)
F5 can be used to force a reload from the server in a local environment. But over the net you would have to do a 'hard' refresh CTRL + F5. This is on IE, Netscape would be sonmething similar.
This causes the browser to go back to the originating server to refresh the request, and not take a copy from a cache.
Just my interpretation, the terminology could be different, but the theory AFAIK is the same.
When you resubmit with the data in the form, its a new request as far as the server is concerned, hence twice the data
HTH
;P