def process_item(self, item, spider):  
url = self.clean_url(item["<STR_LIT:url>"])<EOL>if self.is_sequence_start_page(url):<EOL><INDENT>url = url.parent<EOL><DEDENT>if url in self.urls_seen:<EOL><INDENT>raise DropItem(u"<STR_LIT>".format(url=item["<STR_LIT:url>"]))<EOL><DEDENT>else:<EOL><INDENT>self.urls_seen.add(url)<EOL>return item<EOL><DEDENT>
Stops processing item if we've already seen this URL before.