title: Django auth.user with unique email
body: <p>I use the django.auth system and I've this:</p>
  
  <pre><code>class RegisterForm(UserCreationForm):
      username = forms.RegexField(label= "Username" , max_length = 30, regex = r'^[\w]+$', error_messages = {'invalid': "This value may contain only letters, numbers and _ characters."})
      email = forms.EmailField(label = "Email")
      first_name = forms.CharField(label = "First name", required = False)
      last_name = forms.CharField(label = "Last name", required = False)
  
      class Meta:
          model = User
          fields = ("username", "first_name", "last_name", "email", )
  
      def save(self, commit = True):
          user = super(RegisterForm, self).save(commit = False)
          user.first_name = self.cleaned_data["first_name"]
          user.last_name = self.cleaned_data["last_name"]
          user.email = self.cleaned_data["email"]
          if commit:
              user.save()
          return user
  </code></pre>
  
  <p>I want to set emails as uniques and check the form for this validation. How can I do it?</p>