events.js:182
      throw er; // Unhandled 'error' event
      ^

Error: Can't set headers after they are sent.
    at validateHeader (_http_outgoing.js:489:11)
    at ServerResponse.setHeader (_http_outgoing.js:496:3)
    at ServerResponse.header (D:hourmintnode_modulesexpresslibresponse.js:7
30:10)
    at ServerResponse.location (D:hourmintnode_modulesexpresslibresponse.js
:847:15)
    at ServerResponse.redirect (D:hourmintnode_modulesexpresslibresponse.js
:885:18)
    at D:hourmintapproutes.js:298:18
    at D:hourmintnode_modulesmongooselibquery.js:2917:18
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)

var smtpTransport = nodemailer.createTransport({
  service: 'Gmail',
  auth: {
    user: 'noemail@gmail.com',
    pass: 'mypassword'
  },
});

app.post('/addresources', isLoggedIn, function(req, res) {
  var id = (req.body._id) ? req.body._id : mongoose.Types.ObjectId(0);
  var newPwd = new mongo.resource();

  var pwd = newPwd.generateHash(req.body.res_password);
  var resourceValue = {
    'role_id': req.body.role_id,
    'privilege_id': req.body.privilege_id,
    'res_fname': req.body.res_fname,
    'res_lname': req.body.res_lname,
    'res_email': req.body.res_email,
    'res_password': pwd,
    'res_empno': req.body.res_empno,
    'res_city': req.body.res_city,
    'res_mobile': req.body.res_mobile,
    'res_emerno': req.body.res_emerno,
    'res_hourlyrate': req.body.res_hourlyrate,
    'res_otp': 0,
    'res_status': req.body.res_status
  };

  mongo.resource.findOne({
    $or: [{
      'res_email': req.body.res_email
    }, {
      'res_empno': req.body.res_empno
    }]
  }).exec(function(err, user) {
    if (err) throw err;
    if (user) {
      console.log(user)
      if (user.res_email == req.body.res_email && user.res_empno == req.body.res_empno) {

        req.flash('error', 'Email And Employee Number Already Exists!!!')
        res.redirect('/resources')


      } //user already exists with email AND/OR phone.
      else if (user.res_email == req.body.res_email) {

        req.flash('error', 'Email Id  Already Exists!!!')
        res.redirect('/resources')


      } //no users with that email NOR phone exist.
      else(user.res_empno == req.body.res_empno) {

        req.flash('error', 'Employee Number Already Exists!!!')
        res.redirect('/resources')

      }
    } else {


      mongo.resource.findOneAndUpdate({
        '_id': id
      }, resourceValue, {
        upsert: true,
        new: true
      }, function(err, response) {
        if (err) throw err;


        console.log("Resource Added SuccessFully")
        console.log(response.res_email)
        res.redirect('/resources')

        var mainOptions = {
          from: '"Niyati" <noemail@gmail.com>',
          to: response.res_email,
          subject: 'Hello ✔', // Subject line
          text: 'Hello world ?', // plaintext body
          html: '<b>Hello world ?</b>' // html body
        };
        //console.log("html data ======================>", mainOptions.html);
        smtpTransport.sendMail(mainOptions, function(err, info) {
          if (err) throw err;
          console.log("mail Sent");
          // console.log(err);
        });

      });
    }
  });
});