Are you using 2.4 or 3?? Looks like in SS3 this is only available on an ArrayList not a DataList...
If SS3 you could try
$cities = DataObject::get('Ilan', null, 'Sehir ASC', null, null, 'ArrayList');
It would be best to push all this back into a DB query if possible, with something like GROUP BY Sheir. Other wise it's just loading unnecessary stuff into memory + SQL will be much fast at sorting out the duplicates.