I've installed latest DokuWiki under IIS on Windows 2000, using PHP 4.3.6.
Whenever I edit a page and save it (using latest Firefox or IE), I always get a conflict report but with a blank diff. If I then click "Cancel" or "Show page", it is clear that the page actually saved correctly.
This behaviour would be consistent with the form getting submitted twice.
The bug is in doku.php where the save action sends the "Location" header. This change fixes the problem for me:
// header("Location: ".wl($ID));
header("Location: ".wl($ID, '','doku.php',true));
Note: HTTP/1.1 requires an absolute URI as argument to Location: including the scheme, hostname and absolute path, but some clients accept relative URIs. You can usually use $_SERVER['HTTP_HOST'], $_SERVER['PHP_SELF'] and dirname() to make an absolute URI from a relative one yourself: