title: How to avoid storing duplicate results
body: <p>I store scraped content within a csv file.
  Each row contains a unique ID and description of an item.</p>
  <p>My ID is coming from the website where I <strong>scrape</strong> the content and not generated on the scraper side.</p>
  <p>I use Scrapy's feedExporter to generate the csv file</p>
  <p>When I <strong>scrape</strong> again my website, I would like my script to check if unique ID is already stored within the csv file, if it's not I would add the new row, if it is I will just move on to the next item.</p>
  <p>As I assume this is a classic thing to do with a scraping framework I believe there must be a smart way to do it with Scrapy however I can't find anything on this topic within the Scrapy's documentation</p>
  <p>Should I simply open the csv file, go through each item and if iterator's value not present add a new row or skip if it is?</p>