docs: def request_seen(self, request):
          """Returns True if request was already seen.
  
          Parameters
          ----------
          request : scrapy.http.Request
  
          Returns
          -------
          bool
  
          """
          fp = self.request_fingerprint(request)
          # This returns the number of values added, zero if already exists.
          added = self.server.sadd(self.key, fp)
          return added == 0