duplicates-2.js

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

const duplicates2 = arr => {
  
  for (let i = 0; i < arr.length; i++) {
    
    // Cut down on our work by not considering pairs
    // we have already looked at
    for (let j = i + 1; j < arr.length; j++) {
      
      // We have a match!
      if (arr[i] === arr[j]) {
        return true
      }
    }
  }
  
  // We looked at everything and no matches! 
  // There are no duplicates
  return false
}

console.log(duplicates2(nums))  // false
console.log(duplicates2(nums2)) // true