The XML Schema for the XML dump format used by MediaWiki has no constraints for
the page and revision identifiers. This can be easally fixed with the attached
patch. Having it enforced in the XSD makes sense, since I think that some
parsers capable of Schema validation can work more efficiently if they're there.
Another reason is that (however unlikely) some other software might output files
in this format are not obliged to keep the IDs unique, according the the XSD in
its current form.