Practice · 1 of 4

First duplicate

CHALLENGE

Difficulty: beginner+25 XP

Implement static int FirstDuplicate(int[] nums) — return the value whose **second occurrence appears earliest** when scanning left to right; -1 if no value repeats. For [2,1,3,1,2] the answer is 1 (its second occurrence at index 3 beats 2's at index 4).

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