PUT request overwrites array values with duplicate values - angularjs

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 });