'If you invite an address already using Zulip, no invitation is sent.'

def test_invite_existing_user(self):

    self.login(self.example_email('hamlet'))
    self.assert_json_error(self.client_post('/json/invites', {'invitee_emails': self.example_email('hamlet'), 'stream': ['Denmark'], 'custom_body': ''}), "We    weren't    able    to    invite    anyone.")
    self.assertRaises(PreregistrationUser.DoesNotExist, (lambda : PreregistrationUser.objects.get(email=self.example_email('hamlet'))))
    self.check_sent_emails([])
