title: How should I validate and manage a login namespace?
body: <p>This is silly, but I haven't found this information. If you have names of concepts and suitable references, just let me know.</p>
  
  <p>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.</p>
  
  <p>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.</p>