Check for duplicate scraper class names.

def check_scrapers():
    """Check for duplicate scraper class names."""
    d = {}
    for scraperclass in _scraperclasses:
        name = scraperclass.getName().lower()
        if name in d:
            name1 = scraperclass.getName()
            name2 = d[name].getName()
            raise ValueError('duplicate scrapers %s and %s found' % (name1, name2))
        d[name] = scraperclass