FindingDuplicates

import java.util.*;
class Main {
   public static void main(String[] args){
       String[] input = {"INC1001", "INC1002", "INC1003", "INC1001", "INC1004", "INC1002"};
        System.out.println(new Main().findDuplicates(input));
    }
    
    public List<String> findDuplicates(String[] tickets){
        List<String> list = new ArrayList<>();
        Set<String> set = new HashSet<>();
        List<String> outputList = new ArrayList<>();
        
        for(String ticket: tickets){
            list.add(ticket);
        }
        for(String ticket: list){
            if(!set.add(ticket)){
                outputList.add(ticket);
            }
        }
        
        return outputList;
        
    }
}