void remover_letras_duplas(char *str);

antes.c

void
remover_letras_duplas(char *str)
{
  unsigned char cur, ult = 0;
  char *ptr, *tmp, *ptmp;

  ptr = str;
  tmp = malloc(strlen(str) + 1);
  ptmp = tmp;

  while (*ptr)
  {
    cur = *ptr;
    if (cur != ult)
    {
      *ptmp = cur;
      ptmp++;
    }
    ult = cur;
    ptr++;
  }

  *ptmp = '\0';
  strcpy(str, tmp);
  free(tmp);
}

depois.c

void
remover_letras_duplas(char *str)
{
  char *pstr = str, *pnew = str;
  char last = *pstr;

  while (*pstr)
  {
    if (last != *pstr)
    {
      pnew++;
      last = *pstr;
      *pnew = last;
    }
    pstr++;
  }

  pnew++;
  *pnew = '\0';
}