def clean_email(self):
        """ 
        Validate that the email is not already registered with another user.
        
        """
        if self.cleaned_data['email'].lower() == self.user.email:
            raise forms.ValidationError(_(u"You're already known under "
                                                "this email address."))
        if get_user_model().objects.filter(
                email__iexact=self.cleaned_data['email']).exclude(
                    email__iexact=self.user.email):
            raise forms.ValidationError(_(u'This email address is already '
                        'in use. Please supply a different email address.'))
        return self.cleaned_data['email']
Validate that the email is not already registered with another user.