Check if object attribute in an array is unique

As a new JS programmer, I wanted to solve this problem using only JS. Here is my attempt, please suggest if I'm missing something over here

Given Object array (or store)

var storeWithDup = [{category: "Cat A"}, {category: "Cat B"}, {category: "Cat A"}]; var storeWithoutDup = [{category: "Cat A"}, {category: "Cat B"}, {category: "Cat C"}];

1. Sort Object array using custom comparison function

var myComparator = function(value1, value2){
    if(value1.hasOwnProperty("category") &&
        value2.hasOwnProperty("category")){

        if(value1.category < value2.category){
            return -1;
        } else if(value1.category > value2.category){
            return 1;
        } else {
            return 0;
        }
    }
};

2. Check for duplicate adjacent elements in an array

var arrayDuplicateChecker = function(inpArray){
    for(var i = 1; i < inpArray.length; i++){
        if(inpArray[i-1].category == inpArray[i].category){
            return true;
        }
    }
    return false;
};

3. Helper function to check for duplicate categories in a store

var checkForDuplicateCat = function(inpStore){
    // 1. Sort array
    inpStore.sort(myComparator);
    // 2. check for adjacent duplicates
    return arrayDuplicateChecker(inpStore);
};

Result

alert(checkForDuplicateCat(storeWithDup));  // True
alert(checkForDuplicateCat(storeWithoutDup));  // False

Link to JS Fiddle: Object array sort and comparator

My, how complicated we make things.

var array = [{category: 'Cat A'}, {category: 'Cat B'}, {category: 'Cat A }],
    groups = _.groupBy(array, 'category');
    duplicated = Object.keys(groups).filter(function(cat) {
        return groups[cat].length>1; 
    });

console.log(duplicated);