That was me who changed your username, I saw it and changed it before ever reading this post. I'm like Highlander there can be only one ;-D.
I will add a wish list item in the project tracker to make it possible to configure it to require unique display names. But I also think it is perfectly reasonable to allow duplicates. It isn't really the username it is only the display name, there is a separate username that is required to be unique currently but it is not displayed because it is used for authentication if you set site settings to not use email for login. So display name is purely for display and if there are fifty John Smiths who all want to be listed that way it seems reasonable to allow it. But I can see the case for wanting to be able to require it to be unique as well for some scenarios.
For actively managed sites such as this one it really isn't a problem. Site owners should manage their sites also for unsavory display names, so a site should be monitored if display names matter to you. There are currently some settings in Web.config that can be used to enforce a regular expression to limit certain kinds of display names but really active monitoring of the site is the best way to keep things as you would like them. If I had believed for one second that another real Joe Audette had registered on this site I would have been more hesitant to change his display name, just the luck of a less common name I guess ;-D