Checking array items against another array

You are adding currentItem in each iteration where it's not found. filteredItems.push(currentItem); must be called after the loop if not found:

...

    var found = false; // new 'found' var = true if currentItem is found
    for (var x = 0; x < filteredItems.length; x++) {
        if (filteredItems[x].OMSID == currentItem.OMSID) {
            // match found
            found = true;
            break;
        }
    }

    // match not found, add to filtered items.
    if (!found) {
        filteredItems.push(currentItem);
    }

} else {
    filteredItems.push(adlet.item[i]);
    // console.log(filteredItems.length);
}

...

/* The array method 'indexOf' is too useful to ignore, if you do anything with arrays. There is a shim here, based on Mozilla's code.

The method 'add' shown here works like push, but it only adds an item if it is not in the array already. You can add multiple items with multiple arguments.

'merge' will add all 'new' items from the calling array to the array sent as the argument, */

if(!Array.prototype.indexOf){
    Array.prototype.indexOf= function(what, i){
        i= i || 0;
        var L= this.length;
        while(i< L){
            if(this[i]=== what) return i;
            ++i;
        }
        return -1;
    }
}
Array.prototype.add= function(){
    var what, a= arguments, i= 0, L= a.length;
    while(i<L){
        what= a[i++];
        if(this.indexOf(what)=== -1) this.push(what);
    }
    return this;
}
Array.prototype.merge= function(target){
    return this.add.apply(target, this);
}

var a1= [1, 2, 3, 4, 5, 6], a2= [2, 4, 6, 8, 10];
a1.merge(a2)
/*  returned value: (Array)
2, 4, 6, 8, 10, 1, 3, 5
*/

Here's a simplified version of your function, that only adds the filter item after the inner loop finishes:

function FilterItems(attrName, attrValue, priceMin, priceMax) {
   for (var i = 0; i < adlet.item.length; i++) {
      if (adlet.item[i][attrName] == attrValue) {
         var currentItem = adlet.item[i];
         var found = false;
        for (var x = 0; x < filteredItems.length; x++) {
           if (filteredItems[x].OMSID == currentItem.OMSID) {
              found = true;
              break;
           }
        }
        if (!found)
           filteredItems.push(currentItem);
      }
  }
}