How to find duplicates element from a list?

This is an interview question one of my colleague asked, so thought of sharing.

Below is code snippet

List<String> stringList = new List<String>{'One','two','Three','Four','One','two'};

set<String> setString = new set<String>();
set<String> duplicatesetString = new set<String>();
for(String s:stringList ){
   if(!setString.add(s)){
      duplicatesetString .add(s);
   }
}
System.debug('duplicatelistString----'+duplicatesetString );
System.debug('duplicatelistString----'+duplicatesetString.add('ten'));