如何避免mongoose中的重复条目

const userSchema = new mongoose.Schema({
  email: { type: String, required: true, unique: true },
  // 其他字段...
});

const User = mongoose.model('User', userSchema);

User.findOneAndUpdate(
  { email: 'example@example.com' },
  { name: 'John Doe', email: 'new@example.com' },
  { upsert: true, new: true },
  (err, user) => {
    if (err) {
      console.error(err);
    } else {
      console.log(user);
    }
  }
);

const email = 'example@example.com';

User.findOne({ email }, (err, user) => {
  if (err) {
    console.error(err);
  } else if (user) {
    console.log('该email已存在');
  } else {
    const newUser = new User({ email, name: 'John Doe' });
    newUser.save((err, savedUser) => {
      if (err) {
        console.error(err);
      } else {
        console.log(savedUser);
      }
    });
  }
});