When I try to create two posts or page that have same slug I got a warning says to change the slug. However the save/draft/publish button are not disabled. Submitting one of this will generate a database error.
Probably some lines of javascript will solve the bug
Francesco, I tried this with a blog entry and it works as there is code that double checks the slug and appends a unique hash at the end if they are the same:
Example: disk-queues-77CAF