Longest duplicate substring

Given a string s of lowercase letters (1 <= len(s) <= 3*10^4), return the longest substring that occurs at least twice in s (occurrences may overlap). If no character repeats such that a duplicated substring exists, return the empty string. If several answers of the same maximal length exist, returning any one of them is acceptable, but for these tests return the FIRST (lowest second-occurrence index) found by the standard binary-search-plus-rolling-hash method.