MongoError: E11000 duplicate key error collection: timeformationBD.formations index: description_1 dup key: { : null }

hello , plz any one help me
i am getting duplicate key error while doing the POST action i tried several to correct them but I do not understand what is happening

Error MongoError: E11000 duplicate key error collection: timeformationBD.formations index: description_1 dup key: { : null }

this is my formation.route

 var express = require('express');
    var router = express.Router();
    var Formation = require('../models/formations');

    router.post('/', function(req, res){
        var newFormation = {
            name: req.body.name,
            position : req.body.position,
            department : req.body.department,
            salary: req.body.salary
        }
         Formation.addFormation(newFormation,function(err,formation){
             if(err) throw err;
             res.json(formation);
         });
     })

     module.exports = router

;

this is my formation.models

var mongoose = require('mongoose');
var FormationSchema = new mongoose.Schema({
    name: String,
    position : String,
    department : String,
    salary : String
})

var Formation = module.exports = mongoose.model('Formation', FormationSchema);


module.exports.addFormation = function(newFormation, callback){
    Formation.create(newFormation, callback);
}

this is my app.js ( endpoint )

const express = require('express');
const path = require('path');
const bodyParser = require('body-parser');
const cors = require('cors');
const passport = require('passport');
const mongoose = require('mongoose');
const config = require('./config/database');

const User = require('./models/user');
const Formation = require('./models/formations');



mongoose.connect(config.database ,  { useNewUrlParser: true });
mongoose.connection.on('connected', () => {
console.log('connected to database... ' + config.database);
});

mongoose.connection.on('error', (err) => {
console.log('database error'+err);
});




/*mongoose.connect('mongodb://localhost:27017/hello', { useNewUrlParser: true });
var db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function() {
    console.log('we are connected!');

});*/



const app = express();
const users = require('./routes/users');
const formations = require('./routes/formations');

//port number
 const port = 3001;

//cors middleware
 app.use(cors({
     origin: 'http://localhost:4200'
 }));
 //passwport middelware
 app.use(passport.initialize());
 app.use(passport.session());
 require('./config/passport')(passport); //pour implémenter fichier passport.js

 //set static folder
 app.use(express.static(path.join(__dirname + '/public')));




//BodyParser middleware
app.use(express.json());
app.use(bodyParser.json());
app.use('/users', users);//hedhiya ki nekteb /users barka f url yemchili direct lel const users = require('./routes/users'); fichier hedhka

app.use('/formations', formations); 

//c un route 
//just meloul 7atineha bch ntastiw beha 
//index route
 app.get('/', function(req, res){
    res.send('invalid endpoint');
});



//start Server
app.use(bodyParser.urlencoded({extended:true}));

 app.listen(port, () => {
    console.log('Server started on port' +port);
 });