Atados - First duplicate array test

first-duplicate.js

// Atados Test - First duplicate
//
// Given an array a that contains only numbers in the range from 1 to a.length,
// find the first duplicate number for which the second occurrence has the minimal index.
// In other words, if there are more than 1 duplicated numbers, return the number for which
// the second occurrence has a smaller index than the second occurrence of the other number does.
// If there are no such elements, return -1.
// 
// Example
// 
// For a = [2, 1, 3, 5, 3, 2], the output should be firstDuplicate(a) = 3.
// There are 2 duplicates: numbers 2 and 3. The second occurrence of 3 has a smaller index than the second occurrence of 2 does, so the answer is 3.
// For a = [2, 2], the output should be firstDuplicate(a) = 2;
// For a = [2, 4, 3, 5, 1], the output should be firstDuplicate(a) = -1.
// The element in a that occurs in the array more than once and has the minimal index for its second occurrence.
// If there are no such elements, return -1.

function firstDuplicate(arr) {}