Freeing memory twice is undefined behavior, e.g.
int * x = malloc(sizeof(int)); *x = 9; free(x); free(x);
Quote from standard(22.214.171.124. The free function of C99 ):
Otherwise, if the argument does not match a pointer earlier returned by the calloc, malloc, or realloc function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.