Why does (*p=*p) & (*q=*q); in C trigger undefined behavior

Why does (*p=*p) & (*q=*q); in C trigger undefined behavior if p and q are equal.

int f2(int * p, int * q)
{
  (*p=*p) & (*q=*q);
  *p = 1;
  *q = 2;
  return *p + *q;
}

Source (Nice article by the way): http://ift.tt/O82uIo