How can I extract duplicates from an array using ExtendScript?

I am trying to identify duplicate object names in an Illustrator file using ExtendScript so I can group them. I can get an array of all PageItems on a layer, I just don't know how to identify and store duplicates so I can perform the group operation.

This is a rather pathetic attempt at doing what I want:

var aLayer = app.activeDocument.activeLayer;
var pageItemsRef = aLayer.pageItems;
var duplicateNames = [];
var currentName;

for (var i = 0, len = pageItemsRef.length; i < len; i++) {
    if(i==0){
        currentName = pageItemsRef[0].name;
        continue;
    }
    if(pageItemsRef[i].name == currentName){
        var ref = {name:pageItemsRef[i].name, index:i}
        duplicateNames.push(ref);
    }
}

Continuing with this code will only give me consecutive duplicates. Sorry for the low quality example but I just don't know where to start.

I just want to iterate pageItemsRef and store the name + ID into separate arrays.

So array [0] will have 2+ duplicates stored as objects (Name, ID) Array [1] might have 5+ duplicates stored as objects also stored as (Name,ID) etc.

Any more efficient approaches are warmly welcomed as well. I am new to ExtendScript.

Any help greatly appreciated.

I'd propose this classic solution via the intermediate object obj. Since an object can't have keys with the same name it can be used as a 'filter' to gather into a value the items with the same property (a name in this case):

var layer = app.activeDocument.activeLayer;
var items = layer.pageItems;

// the intermediate object {name1:[items], name2:[items], ...}
var obj = {};
for (var i=0; i<items.length; i++) {
    var item = items[i];
    var info = {name:item.name, id:item.uuid};  // <-- ID is .uuid ???
    if (item.name in obj) obj[item.name].push(info);
    else obj[item.name] = [info];
}

// make the 2d-array from the keys of the intermediate object
// [ [{name1,id1},{name1,id2}], [{name2,id3},{name2,id4},...], ... ]
var duplicates = [];
for (var key in obj) duplicates.push(obj[key]); // <-- here we go

// probably you don't even need the array
// perhaps you can use the obj instead

The only thing I don't get: what do you mean by 'ID'? In my code I'm using the native uuid property of a page item. Or do you mean just the plain counter (index)?