Display Names are as far as I know CaPs SeNsItIvE. As in they can have the same name, But with an assortment of capital letters that the original name holder does not. Also you can change the option to change a name after they have registered. Admin Panel > Settings > Membership and registration > Click allow members to change their profile names.
Hope this helps. Best answer I got
... what he means is that i can register on your site as rg Ninja. but if i register as Beans then try to change my name to rg Ninja it won't allow me because that name is taken. I think by having this post in the feature requests you are asking for the ability to check if a username is taken or not upon registration??