All of this indicates that IE is caching the results of the PHP page. Request that the browser does not by adding this to the top of the PHP:
header("Cache-Control: no-cache, must-revalidate");
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT");
If it works in all other browsers, then this issue is with IE, and not that of the code in use. If it were all browsers, and still persists with no-cache, then its cached at the ISP or router land, in which case there is little that can be done.
I don't understand what you mean by 'double set of file page'? As with the Fahrenheit and Celsius, you do not need multiple pages, you just need to accept input from the user and determine which to show, and default to one or the other when nothing else is available (probably not required as the service this is coming from will do this themselves). This can be done using GET or POST.