How to find duplicates element from a list in Salesforce Apex

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

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