Powerful SEF Control
Flexible and powerful Joomla Core SEF control
Looks like Joomla 3.4 is using this method now for url duplicates. :-)
StackIdeas's EasyDiscuss has it pretty spot on. Basically it has id-less url's. In the case of the alias being the same as another, it simply puts a 2 at the end of the url, or a 3 if a 2 already exist.
So the structure for duplicates could be mysite.com/content/article-title-2
It's not that often that duplicates happen. A little number added to the alias would work just fine for those cases. As it is, you cannot have a article with the same alias as the other. Theoretically this can act as the unique identifier and would also solve the issue of people "faking" your url's for bad reasons.
Nathan Hawks commented
The near-term goal should be, in my opinion, the simplest possible set of horizontal field elements, offering an editable field a la the current Alias field, plus a choice between available URL (nav) contexts. IMO, this also indicates the need for an element to also create a menu entry, in the process of creating a URL, perhaps via a choice of "New" under the selector for the URL (nav) context.
Giu Tae Kim commented
This is the missing chapter of the cms joomla... please consider a powerfull SEF control, with strong multilanguage capabilities