def METHOD_NAME(self):
    """
    Make sure only one submission can be published for each message.
    """
    publish = self.cleaned_data['publish']
    if publish and not self.errors:
        message = self.cleaned_data['message']
        qs = Submission.objects.filter(publish=True, message=message)
        if self.instance:
            qs = qs.exclude(pk=self.instance.pk)
        if qs.exists():
            raise forms.ValidationError(_(
                'This message has already been published in some '
                'other submission. Messages can only be published once.')
            )
    return publish