find-duplicate-number.js

// validate input array elements
function isIntegerArray (input) {
  // if an array
  if (Array.isArray(input)) {
    // if empty array
    if (input.length < 1) return false

    // iterate through each element and return false if one of them happens to be not an integer
    for (let i = 0; i < input.length; i++) {
      if (Number.isInteger(input[i])) continue
      else return false
    }
    return true
  // if not an array
  } else return false
}

function findDuplicateNumber (input) {
  if (isIntegerArray(input)) {
    // for integers being compared
    for (let i = 0; i < input.length; i++) {
      // for integers being compared with
      for (let j = 0; j < input.length; j++) {
        // if not comparing to self
        if (!(i === j)) {
          // AND if found similar with each other, return integer
          if (input[i] === input[j]) return input[i]
        }
      }
    }
  } else return 'Please input an array of integers.'
}

console.log(findDuplicateNumber([1, 68, 3, 4, 5, 68, 9, 10]))