在JavaScript中仅向数组添加唯一对象

Let's say I start with this:

让我们说我从这开始：

var shippingAddresses =      [{
                                "firstname": "Kevin",
                                "lastname": "Borders",
                                "address1": "2201 N Pershing Dr",
                                "address2": "Apt 417",
                                "city": "Arlington",
                                "state": "VA",
                                "zip": "22201",
                                "country": "US"
                            }, {
                                "firstname": "Dan",
                                "lastname": "Hess",
                                "address1": "304 Riversedge Dr",
                                "address2": "",
                                "city": "Saline",
                                "state": "MI",
                                "zip": "48176",
                                "country": "US"
                            }];
var