I suggest next code for 'xmlNormalizeWindowsPath' function. It is 
necessary for forming equal URL's for same documents.

xmlChar *
xmlNormalizeWindowsPath(const xmlChar *path)
{
    int len, i = 0;
    xmlChar *ret;
	xmlURIPtr uri;

    if (path == NULL)
	return(NULL);

	if((uri = xmlParseURI(path)) != NULL)
	{
		xmlFreeURI(uri);
		return xmlStrdup(path);
	}
	uri = xmlCreateURI();

	uri->path = xmlStrdup(path);

	len = xmlStrlen(uri->path);
    while (i < len) {
	if (uri->path[i] == '\\')
	    uri->path[i] = '/';
	i++;
    }
	ret = xmlSaveUri(uri);
	xmlFreeURI(uri);
    return(ret);
}

I have partially fixed this. 

The first proposition about changing ctxt->myDoc->URL = ... leaks
memory, so I changed it a little bit.

The xmlNormalizeWindowsPath has been obsoleted and the new
xmlCanonicPath now takes over its place. It works similarily to what
you propose above, so I think your problem is solved.

About the actual problem you mentioned, namely that identical
documents are being parsed multiple times, that has not been solved
with the above, and I doubt that can be solved in a short term. I'll
therefore assign this report to pending, the problem needs a general
analysis.

For the first change:
Please check, that your change works!!! After this change I have a 
new bug - if filename is non-wellformed (for example spaces in path) 
it is not works. In other words filename must be parseable as URI.
For this requirement I make changes in xmlNormalizeWindowsPath, it 
solve problems in my application, but it is not solve problem in 
library.
I suppose, that solve of problem requires accurate separation between 
URL's and paths in whole library.

This problem I see then I try preload big XML-document, which uses in 
several transformers.
I load document using xmlParseFile() and place it in transformer 
using xsltNewDocument. And as result the doc->URL mismatchs with URL 
of document loaded by xsl:document() function durning transformation.

This should be closed in libxml2 release 2.5.4 and libxslt release 1.0.27,

  thanks,

Daniel