This is silly, but I haven't found this information. If you have names of concepts and suitable references, just let me know.
I'd like to understand how should I validate a given named id for a generic entity, like, say, an email login, just like Yahoo, Google and Microsoft do.
I mean... If you do have an user named foo, trying to create foo2 will be denied, as it is likely to be someone trying to mislead users by using a fake id.