Cannot set headers after they are sent to the client but data is being sent

I am new to express.js and I was sending data of a user using mongoose to send data to mongoDB but after I send 1 incorrect data this error pops up and If I try to send a valid user information this error pops up again but the entries are being sent to my mongoDB database.
Case 1:

• if I send a correct entry : It works
• if I send another correct entry : It works
• If I send an incorrect entry : It crashes

Case 2:

• if I send a correct entry : It works
• if I send an incorrect entry : It crashes
• If I send a correct entry : It crashes but it still sends the data to my database

So do I have to reload everytime after it crashes once or there is a better solution to that?

Error:

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at new NodeError (node:internal/errors:387:5)
    at ServerResponse.setHeader (node:_http_outgoing:603:11)
    at ServerResponse.header (D:FilesDesktopProgrammingReactinotebookbackendnode_modulesexpresslibresponse.js:794:10)
    at ServerResponse.send (D:FilesDesktopProgrammingReactinotebookbackendnode_modulesexpresslibresponse.js:174:12)
    at ServerResponse.json (D:FilesDesktopProgrammingReactinotebookbackendnode_modulesexpresslibresponse.js:278:15)
    at Connection.onMessage (D:FilesDesktopProgrammingReactinotebookbackendnode_modulesmongodblibcmapconnection.js:213:9)      
    at MessageStream.<anonymous> (D:FilesDesktopProgrammingReactinotebookbackendnode_modulesmongodblibcmapconnection.js:59:60) 
    at MessageStream.emit (node:events:513:28)
    at processIncomingData (D:FilesDesktopProgrammingReactinotebookbackendnode_modulesmongodblibcmapmessage_stream.js:124:16)  
    at MessageStream._write (D:FilesDesktopProgrammingReactinotebookbackendnode_modulesmongodblibcmapmessage_stream.js:33:9)   
    at writeOrBuffer (node:internal/streams/writable:391:12) {
  index: 0,
  code: 11000,
  keyPattern: { email: 1 },
  keyValue: { email: '[email protected]' },
  [Symbol(errorLabels)]: Set(0) {}
}

Code:
index.js –

const connectToMongo = require('./db');
const express = require('express');
// npm run server

connectToMongo();
const app = express();
const port = 3000;

app.use(express.json());

// * Available Routes
app.use('/api/auth', require('./routes/auth'));
app.use('/api/notes', require('./routes/notes'));


app.listen(port, () => {
  console.log(`Example app listening on port at http://localhost:${port}`)
})

db.js –

const mongoose = require('mongoose');

const mongoURI = 'mongodb://localhost:27017/iNotebook';

const connectToMongo = () =>{
  mongoose.connect(mongoURI).then(()=>{
    console.log('Connected to Mongo Successfully');
  });
}

module.exports = connectToMongo;

routes/auth.js –

const express = require('express');
const router = express.Router();
const User = require('../models/User');
const { body, validationResult } = require('express-validator');



// * Create A User using: POST "/api/auth/". Doesn't require Auth
router.post('/',[
  body('name', 'Enter a valid name').isLength({min: 3}),
  body('email', 'Enter a valid email').isEmail(),
  body('password', 'Password must be atleast 5 characters').isLength({min: 5})
], (req, res)=>{
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  User.create({
    name: req.body.name,
    email: req.body.email,
    password: req.body.password,
  }).then(user => res.json(user))
  .catch(err => console.log(err),
  res.json({error: 'Please enter a unique value for Email'}))
});
module.exports = router;

routes/notes.js –

const express = require('express');
const router = express.Router();

router.get('/', (req, res)=>{
  
  res.json([]);
});

module.exports = router;

models/Notes.js –

const mongoose = require('mongoose'); 

const NotesSchema = new Schema({
  title:{
    type: String,
    required: true
  },
  description:{
    type: String,
    required: true
  },
  tag:{
    type: String,
    default: "General"
  },
  date:{
    type: Date,
    default: Date.now
  }
});

module.exports = mongoose.model('notes', NotesSchema);

models/User.js –

const mongoose = require('mongoose'); 
const { Schema } = mongoose;


const UserSchema = new Schema({
  name:{
    type: String,
    required: true
  },
  email:{
    type: String,
    required: true,
    unique: true
  },
  password:{
    type: String,
    required: true
  },
  date:{
    type: Date,
    default: Date.now
  }
});

const User = mongoose.model('user', UserSchema);
User.createIndexes();
module.exports = User;