This has been a problem ever since the first days of the Web.  There's not much you can do about it.  One possibility is to run your application in a new window that has no toolbar and menu bar (like this forum), but the user can still right-click on the page and select back from the context menu.

 

Another possibility to avoid data duplication is to add a hidden field to your form and populate it with a randomly generated string.  You can then create a table that stores these strings whenever your script makes changes to your database as a result of a form submition, but only if the currently submitted random string is not already in the table.  A safe way to create these random strings is to use SYS(2003) + LTRIM(STR(Request.CurrentChannel)).