Detect Duplicate Page URL Segments

[mad-moo]

When creating multiple pages, either via @rio.page, rio.ComponentPage or rio.Redirect, it is possible for multiple pages to share a common URL segment. This is nonsensical, because it's not clear which one should be displayed when navigating to that URL.

This also causes secondary problems e.g. with the default navigation which doesn't expect duplicate URLs and then displays wrong links.

It would be nice to have a check for this, and either raise an exception.