I seem to remember there was a similar post here with what would appear to be an identical issue that concluded it was not the fault of XAMPP but rather a browser issue perhaps???:viewtopic.php?f=16&t=37190
So to start with you may like to provide more detail and paste in between forum code tags the code you are using that gives this issue so that it can be used, possibly, to try and replicate the problem.
One problem at localhost level is browser caching (Temporary Internet Files in IE
) that may be giving a cached page and not a fresh page from the server - to test this theory try and clear the cache (Delete Files... button under Temporary Internet Files in IE
) before posting the data the second time.
If that works then there is usually a setting for the cache that virtually eliminates it by requesting a fresh page from the server every time you visit a page.