为什么会出现此重复错误- Error creating new user: WriteError({"code":11000,"index":0,"errmsg":"insertDocument :: caused by :: 11000 E11000 duplicate key error index？

提供的所有字段都不为空。

架构：

// Declare schema
var userSchema = new mongoose.Schema({
    username: {type: String, required: true, index: {unique: true}},
    password: {type: String, required: true},
    created_on: {type: Date, default: Date.now}
});

发布：

// Create - POST
// Create the first method of the API : POST used to create a new user.
router.post("/", function(req, res, next) {
    // Get values from POST request
    var username = req.body.username;
    var password = req.body.password;
    console.log(req.body); // { username: 'tealou', password: 'test123' }

    // Create new user document
    User.create({
        username: username,
        password: password
    }, function(err, user) {
        console.log(user); // undefined
        if (err) {
            console.log("Error creating new user: " + err);
            res.send("Error creating new user.");
        } else {
            console.log("POST creating new user: " + username);
            res.json(user);
        }
    })
});

错误：

创建新用户时出错：WriteError（{：“ 11000，” index“：0，” errmsg“：” insertDocument ::由::
11000 E11000重复键错误导致：iotdb.users。$ name_1 dup key：{： null}“，” op“：{”用户名“：”
tealou“，”密码“：” $ 2a $ 10 $
7mPGND2FRuJDGnXaVTnkru2.xsGn2Ksf8veBKur4ouD9VUNj60RaC“，” _ id“：”
5786020088245d33140d6f94“：”“创建55：28.279Z“，” __ v“：0}}）

有任何想法吗？