title: Which HTTP response code for "This email is already registered"?
body: <p>I'm creating a RESTful API for creating users that enforces unique email addresses:</p>
  
  <p>Successful <code>POST /users</code>: <code>HTTP 201 Created</code></p>
  
  <p>If I <code>POST</code> the same email address again, what should the response code be? Is <code>409 Conflict</code> the appropriate response code?</p>