Please solve the programming task below using a self-contained code snippet in a markdown code block.

Given a string s, consider all duplicated substrings: (contiguous) substrings of s that occur 2 or more times. The occurrences may overlap.
Return any duplicated substring that has the longest possible length. If s does not have a duplicated substring, the answer is "".
 
Please complete the following python code precisely:
```python
class Solution:
    def longestDupSubstring(self, s: str) -> str:
```