check unique username when updating user details jquery modal

i'm using jQuery modal for user create and update for my laravel project. when creating new user im checking the username availaibilty. but that function doesnot support with update , when user save data without changing username it gives usernam availble error . please advice how to fix

my controller,

public function checkUsernameAvailability() {

$user = DB::table('users')->where('username', Input::get('username'))->count();

if ($user > 0) {
    $isAvailable = FALSE;
} else {
    $isAvailable = TRUE;
}

echo json_encode(
    array(
        'valid' => $isAvailable
    ));

}

jQuery modal

                        username: {
                            validators: {
                                notEmpty: {
                                    message: 'The Username field is required'
                                },
                                rules : {
                                    username : { alphanumeric : true }
                                },
                                remote: {
                                    url: "{{ URL::to('/checkUsername') }}",
                                    data: function (validator) {
                                        return {
                                            username: validator.getFieldElements('username').val()
                                        }
                                    },
                                    message: 'The Username is not available'
                                },
                                stringLength: {

                                    max: 100,
                                    message: 'The Username must be  less than 100 characters long'
                                }
                            }
                        },

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