def METHOD_NAME(self):
    groups = self.cleaned_data.get("groups")
    group_list = groups.values_list("id", flat=True)
    group_organisations = Organisation.objects.filter(
        groups__in=group_list
    ).values_list("name", flat=True)
    duplicates = [
        item
        for item, count in collections.Counter(group_organisations).items()
        if count > 1
    ]
    if duplicates:
        count = len(duplicates)
        message = ngettext(
            "User is member in more than one group "
            "in this organisation: %(duplicates)s.",
            "User is member in more than one group "
            "in these organisations: %(duplicates)s.",
            count,
        ) % {"duplicates": ", ".join(duplicates)}
        raise ValidationError(message)
    return self.cleaned_data