takes in 2 arrays. find what is the same for both.

find-duplicates.js

// only list duplicates once
// return array of duplicates

// helper findInstances(array)
  // obj
  // loop through array
    // if no key in obj, create key
  // return obj

// main findSimilar(arr1, arr2) 
  // dupes = []
  // firstObj = helper(arr1);
  // secondObj = helper(arr2)
  // loop through first obj keys
    // if secondObj[key]
      // push key in dups

  // return dupes

function findInstances(arr) {
  let obj = {};
  for (let i = 0; i < arr.length; i++) {
     if (!obj[arr[i]]) {
      obj[arr[i]] = true;
     }
  }
  return obj.
}

function findDupes(firstArr, secondArr) {
  let firstObj = findInstances(firstArr);
  let secondObj = findInstances(secondArr);
  let dupes = [];
  
  for (key in firstObj) {
    if (secondObj[key]) {
      dupes.push(key);
    }
  }
  
  return dupes;
}