Prevent qdoc from reporting duplicate pages for external pages with the same URL and title

qdoc detects that the documentation contains two pages with the same title. For external pages, these are problematic only if the external pages have different URL's, i.e. the same title referring to two different external pages. Currently, qdoc reports the duplicate page warning for external pages even when the URL's are the same. These aren't really errors. qdoc should not report them.