def METHOD_NAME(self, queryset):
    """
    Generate error message when some of the field has more than one value.
    """
    non_unique = {}
    for field in self._invoice_report_common_fields:
        items = queryset.values(field).distinct()
        if items.count() != 1:
            if field == 'invoice_date':
                data = ', '.join(
                    item[field].strftime("%Y-%m-%d")
                    for item in items if item[field]
                )
            else:
                data = ', '.join(
                    item[field] for item in items if item[field]
                )
            non_unique[field] = data
    non_unique_items = ' '.join([
        '{}: {}'.format(key, value)
        for key, value in non_unique.items() if value
    ])
    return '{}: {}'.format(
        _('Selected items have different'), non_unique_items,
    )