[Bebarr Swallow]

function uniqueData(a) {
  var cache = [];
  
  for(var x = 0; x < a.length; x++) {
    if(x == 0) {
      cache[x] = a[x];
    } else {
      for(var y = 0, alertDublicate = false; y < cache.length; y++) {
        if(cache[y] == a[x]) alertDublicate = true;
        if(y + 1 == cache.length && alertDublicate != true) cache.push(a[x]);
      };
    };
  };
  return cache;
};

uniqueData([1,2,2,3,9,9,2,5,7,8]);

Первая попытка.