const express = require('express');
const router = express.Router();
const mongoose = require('mongoose');
const _ = require('underscore');

let sampleSchema = new mongoose.Schema({
  name: {
    type: 'String',
    unique: true
  },
  tags: [{
    type: 'String'
  }]
})
sampleSchema.pre('save', function (next) {
  this.tags = _.uniq(this.tags);
  next();
});
const Sample = mongoose.model('sample', sampleSchema, 'samples');

router.post('/sample', function (req, res, next) {
  const sample = new Sample(req.body);

  sample.save()
    .then((sample) => {
      return res.send(sample);
    })
    .catch(err => {
      return res.status(500).send(err.message);
    })
});

我得出的结论是，这是不可能通过 Mongoose 模式。
JSON模式是这样完成的。

let schema = {
   name: { type: 'string' }
   tags: { 
      type: 'array',
      items: { type: 'string', uniqueItems: true }
   }
}

在创建Mongo文档之前，我将使用JSON模式进行验证。

这个方法建立在Med的答案上，处理引用，并且完全在方案验证中完成。

let sampleSchema = new mongoose.Schema({
    strings: [{type: 'String'}],
    references: [{type: mongoose.Schema.Types.ObjectId, ref: 'Reference'],
});

sampleSchema.pre('save', function (next) {

    let sample = this;

    sample.strings = _.uniq(sample.strings, function(i) {return (i._id) ? i._id.toString() : i;});
    sample.references = _.uniq(sample.references, function(i) {return (i._id) ? i._id.toString() : i;});

    return next();

});

我有点晚了，但也许这对将来的人有帮助。

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  name: {
    type: String,
  },
  reference: {
    type: [mongoose.Schema.Types.ObjectId],
    ref: 'SomeOtherSchema',
    // Add a custom validator.
    validate: {
      // The actual validator function goes here.
      // "arr" will be the value that's being validated (so an array of 
      // mongoose new ObjectId statements, in this case).
      validator: arr => {
        // Convert all of the items in the array "arr", to their string
        // representations.
        // Then, use those strings to create a Set (which only stores unique
        // values).
        const s = new Set(arr.map(String));
        // Compare the Set and Array's sizes, to see if there were any
        // duplicates. If they're not equal, there was a duplicate, and
        // validation will fail.
        return s.size === arr.length;
      },
      // Provide a more meaningful error message.
      message: p => `The values provided for '${ p.path }', ` +
                    `[${ p.value }], contains duplicates.`,
    }
  },
});

上面的注解代码应该是非常不言自明的。

在MongoDB的更新版本中，当且仅当新值与数组中的元素相比是唯一的时，才可以使用$addToSet追加到数组中。
参考资料如下：https://www.mongodb.com/docs/manual/reference/operator/update/addToSet/
下面是一个例子：

const SampleSchema = new mongoose.Schema({
    tags: [String]
});

const Sample = mongoose.model('Sample', SampleSchema);

// append to array only if value is unique
Sample.findByIdAndUpdate({_id: 1, {$addToSet: {tags: "New Tag"}}});

如果tags数组中还没有"新标签"，这将有效地更新标签。否则，不执行任何操作。