code: function isDuplicate (assetToCheck, isHost) {
    if (parsedAssets.length <= 0) {
      return false
    }
    return ~parsedAssets.findIndex(function (asset) {
      if (isHost) {
        //  We don't want to preconnect twice, eh?
        return asset.split('//')[1] === assetToCheck.split('//')[1]
      }
      return asset === assetToCheck
    })
  }
docs: Checking duplicates is necessary for dns-prefetch and prefetch
  resource hints since the same web page could have multiple assets
  from the same external host, but we only want to dns-prefetch an
  external host once.
   
  Check for duplicates in parsedAssets helper array
  @param {string} assetToCheck
  @param {boolean} isHost