Mongodb MongoError, err： E11000 duplicate key error

我有一个这样的MongoDb模式

    var User = new Schema({
    "UserName": { type: String, required: true },
    "Email": { type: String, required: true, unique: true },
    "UserType": { type: String },
    "Password": { type: String }
});

我正在try 创建一个新用户

    controller.createUser = function (req, res) {

    var user = new models.User({
        "UserName": req.body.UserName.toLowerCase(),
        "Email": req.body.Email.toLowerCase(),
        "UserType": req.body.UserType.toLowerCase()
    });
    models.User.findOne({ 'Email': user.Email }, function (err, olduser) {
                    if (!err) {
                        if (olduser) {
                            res.send({ 'statusCode': 409, 'statusText': 'Email Already Exists' });
                        }
                        else if (!olduser) {
                            user.setPassword(req.body.Password);
                            user.save(function (err, done) {
                                if (!err) {
                                    console.log(user);
                                    res.send({ 'statusCode': 201, 'statusText': 'CREATED' });
                                }
                                else {
                                    res.send({ 'Status code': 500, 'statusText': 'Internal Server Error' });
                                }
                            });
                        }
                    }
                    else {
                        res.send({ 'statusCode': 500, 'statusText': 'ERROR' });
                    }
                });
};

为了创建新用户，我给出了如下属性和值:

 {
"UserName": "ann",
"Email": "ann@ann.com",
"UserType": "normaluser",
"Password":"123456"
}

我得到了这样的错误:

{"Status code":500,"statusText":"Internal Server Error","Error":{"name":"MongoError","err":"E11000 duplicate key error index: medinfo.users.$UserName_1  dup key: { : \"ann\" }","code":11000,"n":0,"connectionId":54,"ok":1}}

我知道这个错误是因为用户名是重复的，但是I haven't set UserName with unique constraint.Whenever I add a new row,I need only email to be unique,UserName can be repeated.如何做到这一点？？