Print Duplication

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Scanner;

public class Duplicate {
  public static void main(String[] args) {
    String source = "1 2 2 2 3 3 4 4 4 5 2 2 2";
    m1(source);
    m2(source);
  }

  private static void m1(String source) {
    Scanner in = new Scanner(source);
    Deque<String> not_duplicate = new ArrayDeque<>();
    Deque<String> duplicate = new ArrayDeque<>();
    while (in.hasNextInt()) {
      String current = Integer.toString(in.nextInt());
      if (not_duplicate.isEmpty() || !current.equals(not_duplicate.getLast())) {
        not_duplicate.addLast(current);
        continue;
      }
      if (duplicate.isEmpty() || !current.equals(duplicate.getLast())) {
        duplicate.addLast(current);
        continue;
      }
    }
    in.close();
    System.out.println(String.join(" ", duplicate));
  }

  private static void m2(String source) {
    Scanner in = new Scanner(source);
    String out = "";
    Integer prev = null;
    Integer last_printed = null;
    while (in.hasNextInt()) {
      int current = in.nextInt();
      if (prev == null || current != prev) {
        prev = current;
        continue;
      }
      if (last_printed == null || current != last_printed) {
        if (last_printed != null) {
          out += " ";
        }
        out += current;
        last_printed = current;
        continue;
      }
    }
    in.close();
    System.out.println(out);
  }
}