PUT request overwrites array values with duplicate values

Question

I am making a small polls app with Yeoman's angular-fullstack generator. I have got to the stage where I am implementing user selection to increment a polls count, which then updates my MongoDB database via a PUT request.

Question is set up ready for the user to vote:

The object has individual IDs:

Then, a user goes to vote on an answer. Looks fine on the client:

But it's not updating correctly in the server. All items in the array change to the first item, with identical IDs:

When the client is refreshed, it loads the data from the server and obviously this is not what I want:

What am I doing wrong?

Here is the view:

<form ng-submit="submitForm()">

    <div ng-repeat="answer in poll.answers">
        <label><input type="radio" name="option" ng-model="radioData.index" value="{{$index}}"/>
            {{ answer.value }} - {{ answer.votes }} Votes
        </label>
    </div>
    <button class="btn btn-success" type="submit">Vote!</button>
</form>

Here is the controller:

'use strict';

angular.module('angFullstackCssApp')
    .controller('ViewCtrl', function ($scope, $routeParams, $http) {
        $http.get('/api/polls/' + $routeParams._id).success(function (poll) {
            console.log(poll);
            $scope.poll = poll;
            $scope.radioData = {
                index: 0
            };

            $scope.submitForm = function () {
                console.log($scope.radioData.index);
                $scope.poll.answers[$scope.radioData.index].votes += 1;
                console.log('scope poll answers:- ', $scope.poll.answers);
                console.log('scope poll answers[index]:- ', $scope.poll.answers[$scope.radioData.index]);
                console.log('votes:- ', $scope.poll.answers[$scope.radioData.index].votes);
                // Change database entry here
                $http.put('/api/polls/' + $routeParams._id, {answers: $scope.poll.answers}).success(function () {
                    console.log('success');
                });
            };
        });
    });

Here is the relevant server-side code, all left from the default of my route and endpoint setting up:

router.put('/:id', controller.update);

// Updates an existing poll in the DB.
exports.update = function(req, res) {
  if(req.body._id) { delete req.body._id; }
  Poll.findById(req.params.id, function (err, poll) {
    if (err) { return handleError(res, err); }
    if(!poll) { return res.status(404).send('Not Found'); }
    var updated = _.merge(poll, req.body);
    updated.save(function (err) {
      if (err) { return handleError(res, err); }
      return res.status(200).json(poll);
    });
  });
};

And the schema:

var PollSchema = new Schema({
    creator: String,
    title: String,
    answers: [{
        value: String,
        votes: Number
    }]
}, { versionKey: false });