First Repeat in JavaScript

Given an array of integers nums, read it from left to right and return the first value that appears for the second time.

If no value appears twice, return -1.

Example 1:

Input: nums = [3, 1, 4, 1, 3]
Output: 1
Explanation: Reading from the left: 3, 1, 4, then 1 again. The second 1 (index 3) comes before the second 3 (index 4), so the answer is 1, although 3 showed up first.

Example 2:

Input: nums = [2, 7, 9]
Output: -1
Explanation: Every value appears once, so nothing repeats.

Example 3:

Input: nums = [6, 6, 6, 6]
Output: 6
Explanation: The second 6, at index 1, is already a repeat.

Constraints:

• nums has at least one number.
• Every number is between 0 and 10⁹, so -1 always means that no value repeats.