Rosebud, you left a support request as a comment. Consider yourself lucky that I’m even looking at it 🙂
There is only one way a diary can have the same name as someone elses, and thats if the first user has requested their account be deleted. It is not possible to have two diaries co-exist on the system with the same username because the database has a unique constraint on that column and trying to add a new user with the same name gives an error at the database level. (Not to mention other things get really weird if someone has the same name).
mysql> insert into users ( user_name, password, real_name, real_email )
-> ( 'delancie', 'gobbldygook', 'Steve Brown', 'firstname.lastname@example.org' );
ERROR 1062: Duplicate entry 'delancie' for key 2
So, even if the new user signup screen is broken and neglects to check, the database won’t allow the same username anyway.
However, if you’d like me to look into it, please mail me at email@example.com (click the address at the bottom if you like) and don’t leave support requests as comments!