docs: @Override
      public void dup()
      {
          if (top < 1)
          {
              throw new EmptyStackException();
          }
          if (top >= stack.length)
          {
              stack = Arrays.copyOf(stack, (int) (stack.length*growFactor));
          }
          top++;
          stack[top-1] = stack[top-2];
      }