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;
}
}
