How to check if Mongo's $addToSet was a duplicate or not

Question

I am using Mongoskin + NodeJS to add new keywords to MongoDB. I want to notify the user that the entry was a duplicate but not sure how to do this.

/*
* POST          


        

Answer 1

i am update a array from Collection with this JSON:

{
    "":""
}

route.js

routes.post("/api/:id", Controller.addOne);

Controller.js

async addOne(req, res) {
    //juryman id to list add
    if (Object.keys(req.body).length === 1) {
      console.log("Size 1");
    }
    await Session.findOneAndUpdate(
      { _id: req.params.id },
      { $addToSet: req.body }
    )
      .then(function(success) {
        res.send("Successfully saved.");
      })
      .catch(function(error) {
        res.status(404).send(error);
      });
  },

I have five arrays in my Collection and this changes the JSON array name-value and updates correctly, the respectively Collection array. This works only for one item.