The Double Invitation

Guests 1 through n each got one invitation — but the printer glitched and one guest got TWO. The RSVP list has n+1 entries. 🖨️

You get n and the RSVP list. Print the double-invited guest.

Example:

Input: n = 5, nums = [3, 1, 4, 5, 2, 4] Output: 4

Walking the list with a seen-set, 4 is the first value already seen — the double-invited guest.