getDuplicatedItemsFromArray.js

const sameAsNextIndex = (item, index, array) => item === array[index + 1]
const sameAsPrevIndex = (item, index, array) => item === array[index - 1]

const isDuplicatedOnSortedMapper =
  (item, index, array) =>
    sameAsNextIndex(item, index, array)
    ? item
    : sameAsPrevIndex(item, index, array)
      ? item
      : null

const getDuplicated =
  list =>
    [...list]
      .sort()
      .map(isDuplicatedOnSortedMapper)
      .filter(item => item)