def METHOD_NAME(tags, approved_tags):
    errors = {}
    existing_tags = []
    for tag in tags.split(','):
        if tag.lower() in approved_tags:
            existing_tags.append(tag)
    no_existing_tags = len(existing_tags)
    if no_existing_tags > 1:
        errors['suggested_tags'] = u'%s already exist' % ', '.join(existing_tags)
        errors['existing_tags'] = existing_tags
    elif no_existing_tags == 1:
        errors['suggested_tags'] = u'%s already exists' % existing_tags[0]
        errors['existing_tags'] = existing_tags
    if len(errors) > 0:
        raise ValidationError(errors)