code: def clean_email(self):
          
          contacts = self.api.lists.contacts(id=self.list_id)['result']
  
          for contact in contacts:
              if contact['email'] == self.cleaned_data['email']:
                  raise forms.ValidationError(
                      _(u'This email is already subscribed'))
  
          return self.cleaned_data['email']
docs: Raise ValidationError if the contact exists.