Mongoose and mongoDB - duplicate key error

I'm trying to use MongoDB and mongoose for my little conversation app in where each user is having it's own collection but I'm getting an error when saving a new conversation:

ERROR: MongoError: insertDocument :: caused by :: 11000 E11000 duplicate key error index: chat.privatemessages.$id dup key: { : ObjectId('54f1a39ff467833f7e794636') }

My Schema:

    var chatPrivateMessagesSchema = mongoose.Schema({
    user: String,
    with:[{
        conversation_with: String,
        messages:[{
            text:String,
            created: {type: Date, default: Date.now}
        }]
    }]
});

var PrivateMessages = mongoose.model('PrivateMessages', chatPrivateMessagesSchema);

Method for saving new conversation:

socket.on ('chat', function (from, to, message, callback) {
    PrivateMessages.find ({}, function (err, users) {
        if (err) throw err;
        for (var i = 0; i < users.length; i++) {
            if (users[i].user == from) {
                var newPrivateMsg = new PrivateMessages ({_id: users[i]._id}, {
                    with: [{
                        conversation_with: to, //if conversation with "to" user does not exist create a new entry
                        messages: [{
                            text: message, //append a new message 
                            created: {type: Date, default: Date.now}
                        }]
                    }]
                });
                newPrivateMsg.save (function (err) {
                    if (err) throw err;
                });
            }
        }
    });
});

So basically the output I'm looking for should be something like this?

{
    "_id": "some id",
    "user": "John",
    "with": [{
        "conversation_with": "Maria",
        "_id": "some id",
        "messages": [{
            "text": "Hellomyfisrtmessage",
            "created": "somedate"
        }, {
            "text": "Secondmessage ",
            "created": "somedate"
        }, {
            "text": "Thirdmessage",
            "created": "somedate"
        }, ]
    }, {
        "conversation_with": "Peter",
        "_id": "some id",
        "messages": [{
                "text": "Hellomyfisrtmessage",
                "created": "somedate"
            }, {
                "text": "Secondmessage ",
                "created": "somedate"
            }, {
                "text": "Third message",
                "created": "some date"
            },

        ]
    }],

}

I'm really struggling how to insert a new entry :( Can anyone help please?