Java Set find duplicate values in Collection

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class Main {
  public static void main(String[] args) throws java.lang.Exception {
    List<String> people = new ArrayList<>();
    people.add("Java");
    people.add("Javascript");
    people.add("C++");
    people.add("HTML");
    people.add("Java");
    Set<String> seen = new HashSet<>();
    for (String p : people) {
      if (seen.add(p)) {
        System.out.println(p + " is new");
      } else {/* www.j av  a2  s . co m*/
        System.out.println(p + " is a duplicate");
      }
    }
  }
}