findDuplicates = (arr) => {
  const lookUpObj = {};
  const dupes = [];

  arr.forEach((item) => {
    if (Object.prototype.hasOwnProperty.call(lookUpObj, item)) {
      dupes.push(item);
    }
    lookUpObj[item] = 0;
  });

  return dupes;
}
finds duplicates in a simple array
@param   {array} arr array of items containing comparable items
@return  {array}     array of second (or more) instance of duplicate items