code: function transform(file) {
      // Parses CSS file and turns it into a \n-separated img URLs.
      var data = file.contents.toString('utf-8');
  
      var matches = [];
      var match;
      while ((match = url_pattern.exec(data)) !== null) {
          var url = match[1];
  
          // Ensure it is an absolute URL (no relative URLs).
          var has_origin = url.search(/(https?):|\/\//) === 0 || url[0] === '/';
          if (!has_origin) {
              var absolute_path = path.join(MKT_CONFIG.CSS_DEST_PATH, url);
              url = '/' + path.relative('src', absolute_path);
          }
  
          if (img_urls.indexOf(url) === -1) {
              matches.push(url);
              img_urls.push(url);
          }
      }
  
      return matches.join('\n');
  }
docs: Keep track of duplicates.