def check_duplicate_slugs(contents):
    """
	check for duplicate slugs
	"""
    slugs = []
    for post in contents:
        slug = ''
        if post.path.startswith('pages/'):
            slug = post.path.split('pages/')[1]
        elif post.path.startswith('posts/'):
            slug = post.path.split('posts/')[1]
        if slug and slug in slugs:
            raise ValueError('Duplicate slug : {}'.format())
