First repeat value

Given a list of integers, scan it from left to right and return the first value you encounter for the second time, in other words, the value whose second occurrence has the smallest index. If no value ever repeats, return -1. For example, in [2, 5, 3, 5, 2] the value 5 completes a repeat at index 3, before 2 does at index 4, so the answer is 5. Aim for a single pass.