About semaphore using in RTX51 Tiny

In http://www.keil.com/support/docs/2568.htm I found code about semaphore:

/*---------------------------------------------------------
---------------------------------------------------------*/
#pragma disable
static char _Xget_semaphore (
  unsigned char semaphore_id)
{
if (sem_tab[semaphore_id].count > 0)
  {
  --sem_tab[semaphore_id].count;
  return (-1);
  }

sem_tab[semaphore_id].waiting_task_bits |=
  (1 << os_running_task_id());

return (0);
}

My question is that:
In RTX51 Tiny System, I have multi-tasks: A and B.
task A already got semaphore, program is running after if (sem_tab[semaphore_id].count > 0) and will descrease sem_tab[semaphore_id].count. At the moment, another task B want to get semaphore, so call get_semaphore. Because count still stay 1 so enter char _Xget_semaphore function. Well, both of task A and B got semaphore now.
I think this supposition is rare, but when it occur, it will make system something wrong.
So, how could a semaphore function prevent the situation occur?
Can be there any protection in code?

Thank you for help.