Longest Repeated Substring Finder

You are given a string S. Write a function that finds the longest substring that occurs at least twice in S. Occurrences may overlap. In the case of multiple substrings of the same maximum length, you can return any one of them. If no such substring exists, return an empty string.

Example:

• Input: banana
• Possible Output: ana (since "ana" occurs twice and is one of the longest repeated substrings)

Constraints:

• The input string S consists of lowercase and/or uppercase letters.
• The length of S is between 1 and 10,000 characters.

Your solution should aim for efficiency given the constraints.