I m not sure what you re trying to do, but maybe you just need to aggregate the data?

var data = [
    { name: "portrait", owner: "Jon" },
    { name: "house", owner: "Jim" },
    { name: "portrait", owner: "Jim" },
    { name: "portrait", owner: "Jane" },
    { name: "cat", owner: "Jane" }
];

var byName = {};

for (var i = 0, l = data.length; i < l; ++i) {
    if (!byName[data[i].name]){
        byName[data[i].name] = [];
    }
    byName[data[i].name].push(data[i].owner);
}

// byName == {portrait: ["Jon", "Jim", "Jane"], house: ["Jim"], cat: ["Jane"]}

var byOwner = {};

for (var i = 0, l = data.length; i < l; ++i) {
    if (!byOwner[data[i].owner]) {
        byOwner[data[i].owner] = [];
    }
    byOwner[data[i].owner].push(data[i].name);
}

Or maybe you like this better:

var data = [
    { name: "portrait", owner: "Jon" },
    { name: "house", owner: "Jim" },
    { name: "portrait", owner: "Jim" },
    { name: "portrait", owner: "Jane" },
    { name: "cat", owner: "Jane" }
];

var byName = [];

for (var i = 0, l = data.length; i < l; ++i) {
    var done = false;
    for (var j = 0, k = data.length; j < k; ++j) {
        if (byName[j] && byName[j].name == data[i].name) {
            byName[j].data.push(data[i].owner);
            done = true;
        }
    }
    if (!done) {
        byName.push({name: data[i].name, data: [data[i].owner]});
    }
}

/*
byName == [
    {name: "portrait", data: ["Jon", "Jim", "Jane"]},
    {name: "house", data: ["Jim"]},
    {name: "cat", data: ["Jane"]}
] */

Because posting random code is fun and you know it!

I d suggest looping through the picture array and adding a flagged member to each object, depending if it s duplicate. See below for an example. It will assign anything with a duplicate with an A and the last duplicate with a B . Anything that is not a duplicate is not flagged.

var picture_array = [
        {label:  portrait , owner: "Jon"},
        {label:  house , owner: "Jim"},
        {label:  portrait , owner: "Jim"},
        {label:  portrait , owner: "Jane"},
        {label:  cat , owner: "Jane"}
    ],
    length = picture_array.length;

//Loop through picture_array

for(var i = 0; i < length; i++) {
    var picture = picture_array[i],
        label = picture.label;

    //If picture has already been flagged, go the the next picture
    if (picture.flagged) {
       continue;
    }

    //Loop through rest of the array to compare duplicate labels
    var picture_a = picture;
    for(var j = i + 1; j < length; j++) {
        var picture_b = picture_array[j];

        //If picture_a matches picture_b then flag both of them appropriately
        if (picture_a.label == picture_b.label) {
            picture_a.flagged =  A ;
            picture_b.flagged =  B ;
            picture_a = picture_b;
        }
    }
}