duplicates-3.js

const nums = [2, 6, 3, 10]
const nums2 = [2, 4, 1, 2]

const duplicates3 = arr => {
  for (let i = 0; i < arr.length; i++) {
    const num = arr[i]
    
    // We take a slice of the remaining Array
    // and see if that includes the num 
    // it must be a duplicate
    if(arr.slice(i + 1).includes(num)) {
      return true
    }
  }
  
  // We looked at everything and no matches! 
  // There are no duplicates
  return false
}

console.log(duplicates3(nums))  // false
console.log(duplicates3(nums2)) // true