Warn about clashes in generated pages

qdoc could warn about .html files that are generated from different sources. This can happen e.g. if you have multiple \page commands with the same value, if there's a \group name that conflicts with a namespace , etc ...

These issues are complex to detect, cause links to such pages work ... they just don't point to the right content. But qdoc should know which pages it's generating within the same module, so it might generate a warning if html pages are generated twice.