If you want to remove the duplicates, there is a very simple way, making use of the Set data structure provided by JavaScript. It’s a one-liner:

const yourArrayWithoutDuplicates = [...new Set(yourArray)]

To find which elements are duplicates, you could use this “array without duplicates” we got, and and remove each item it contains from the original array content:

const yourArray = [1, 1, 2, 3, 4, 5, 5]

const yourArrayWithoutDuplicates = [...new Set(yourArray)]

let duplicates = [...yourArray]
yourArrayWithoutDuplicates.forEach((item) => {
  const i = duplicates.indexOf(item)
  duplicates = duplicates
    .slice(0, i)
    .concat(duplicates.slice(i + 1, duplicates.length))
})

console.log(duplicates) //[ 1, 5 ]

Another solution is to sort the array, and then check if the “next item” is same to the current item, and put it into an array:

const yourArray = [1, 1, 2, 3, 4, 5, 5]

let duplicates = []

const tempArray = [...yourArray].sort()

for (let i = 0; i < tempArray.length; i++) {
  if (tempArray[i + 1] === tempArray[i]) {
    duplicates.push(tempArray[i])
  }
}

console.log(duplicates) //[ 1, 5 ]

Note that this only works for primitive values, not objects. In the case of objects, you need a way to compare them.