I have fixed the problem where a page could be created with a duplicate url. The bug was that we were only creating the friendly url if use url was checked, when we should create it any time an url is provided because you could change that setting back to true at any time. So because we did not create the frioendly url, the url would not work even if it was not duplicated and this caused the possibility of pages with the same url because we only check if we are duplicating friendly urls, we don't check against the mp_pages table. By making sure we always create a friendly url if an url is provided means we will not allow pages to have duplicate urls.
Futhermore, I have changed to hide this Use Url setting by default, if you want to show it, then you can add this to your user.config:
<add key="ShowUseUrlSettingInPageSettings" value="true" />
I left this setting in back in 2005 when I first implemented friendly urls and wasn't sure how well it would work. For less user support issues I decide now to hide it by default since people generally should use SEO friendly urls.
Yes, it is true that we don't update friendly urls, we create them or delete them but we do not change existing ones. In fact it is possible to map multiple friendly urls to the same page from Url Manager if desired.
You say you have duplicate friendly urls in your mp_FriendlyUrls table, are you sure about that? Do they have the same site id? It is possible to use the same friendly urls in each site. We check for duplicates only against the current site.
If you are seeing duplicate friendly urls with the same site id then it would help if you can find steps to produce that problem.
So the main issue you reported is now fixed in svn trunk within 24 hours of your bug report that provided steps to produce the problem. In my opinion this is another example of the excellent support I have provided you for my free product. If you feel somehow this process is not working well enough for you I think you need to adjust your expectations.