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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def process_item(self, item, spider):
 # pylint: disable=unused-argument """ Stops processing item if we've already seen this URL before. """

        url = self.clean_url(item["url"])

        if self.is_sequence_start_page(url):
            url = url.parent

        if url in self.urls_seen:
            raise DropItem(u"Dropping duplicate url {url}".format(url=item["url"]))
        else:
            self.urls_seen.add(url)
            return item