Не удается установить заголовки после их отправки. (ошибка nodejs)

Я новичок в Node.js, и у меня возникают некоторые проблемы.

При попытке открыть 127.0.0.1:3000/bejelentkezes я получил следующую ошибку:

POST /bejelentkezes 500 550.784 ms - 2259
GET /css/404.css 304 2.751 ms - -
1
sikeres bejelentkezes
Error: Can't set headers after they are sent.
    at ServerResponse.setHeader (_http_outgoing.js:371:11)
    at ServerResponse.header (C:\Users\denes\Documents\e_wallet\node_modules\express\lib\response.js:730:10)
    at ServerResponse.send (C:\Users\denes\Documents\e_wallet\node_modules\express\lib\response.js:170:12)
    at done (C:\Users\denes\Documents\e_wallet\node_modules\express\lib\response.js:967:10)
    at Object.exports.renderFile (C:\Users\denes\Documents\e_wallet\node_modules\pug\lib\index.js:412:12)
    at View.exports.__express [as engine] (C:\Users\denes\Documents\e_wallet\node_modules\pug\lib\index.js:455:11)
    at View.render (C:\Users\denes\Documents\e_wallet\node_modules\express\lib\view.js:128:8)
    at tryRender (C:\Users\denes\Documents\e_wallet\node_modules\express\lib\application.js:640:10)
    at Function.render (C:\Users\denes\Documents\e_wallet\node_modules\express\lib\application.js:592:3)
    at ServerResponse.render (C:\Users\denes\Documents\e_wallet\node_modules\express\lib\response.js:971:7)

Это мой код:

var express = require('express');
var router = express.Router();
var tools = require('./adatbazis_v2');

router.get('/', function(req, res, next) {
  res.render('login');
});

router.post('/', function(req,res,next) {
  if (!req.body.email || !req.body.password) {
    console.log('valami nincs kitoltve');
  } else {
      tools.checkUser(req.body.email, req.body.password, function(callback) {
        if(callback == 1){
          console.log("sikeres bejelentkezes");
          res.render('index');
        } else {
          console.log('sikertelen bejelentkezes');
          res.render('login');
        }
      });
    }
});

module.exports = router;

и это функция:

checkUser: function (felhasznalonev, jelszo, callback) {

      connection.connect(function(err) {
        if (err) {
          console.error('error connecting: ' + err.stack);
          return;
        }
      });

      connection.query('SELECT COUNT(*) AS szamlalo FROM felhasznalok WHERE fhsz_email="' +
      felhasznalonev + '" AND fhsz_jelszo= "' + md5(jelszo) +'";', function (error, results, fields) {
        console.log(results[0].szamlalo);
        callback(results[0].szamlalo);
      });

      connection.close();
  }

EDIT 1: я обновляю свой код, но я все еще получаю ошибку.

router.post('/', function(req,res,next) {
  var l_status = 0;
  console.log('l_status: '+l_status);
  if (!req.body.email && !req.body.password) {
    console.log('valami nincs kitoltve');
  } else {
      l_status = tools.checkUser(req.body.email, req.body.password, function(callback) {
        return callback;
      });
      if(l_status == 1)
        res.render('index');
      else
        res.render('login');
    }
});

Ошибка:

GET /bejelentkezes 304 1776.456 ms - -
GET /css/signin.css 304 5.524 ms - -
GET /favicon.ico 200 7.638 ms - 32038
l_status: 0
POST /bejelentkezes 500 728.901 ms - 2259
GET /css/404.css 304 13.212 ms - -
1 (this is checkuser output)

EDIT 2 - Дополнительная информация

App.JS:

var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
var session = require('express-session');

var index = require('./routes/index');
var users = require('./routes/users');
var login = require('./routes/login');
var notfound = require('./routes/error');

var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'pug');
app.set('trust proxy', 10);

app.use(session({
  secret: 'E355A874F571C',
  resave: false,
  saveUninitialized: true,
  cookie: { secure: true }
}))

// Authentication and Authorization Middleware
var auth = function(req, res, next) {
  if (req.session && req.session.user === "denes" && req.session.admin)
    return next();
  else
    return res.sendStatus(401);
};

// uncomment after placing your favicon in /public
//app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use('/', index);
app.use('/rogzit', index);
app.use('/bejelentkezes', login);
app.use('/404', notfound);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  /*var err = new Error('Not Found');
  err.status = 404;
  next(err); */

  req.redirect('/404');
});

// error handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

package.json:

{
  "name": "e_wallett",
  "version": "0.0.0",
  "private": true,
  "scripts": {
    "start": "node ./bin/www"
  },
  "dependencies": {
    "body-parser": "~1.17.1",
    "cookie-parser": "~1.4.3",
    "debug": "~2.6.3",
    "express": "~4.15.2",
    "morgan": "~1.8.1",
    "pug": "~2.0.0-beta11",
    "serve-favicon": "~2.4.2"
  }
}

Adatbazis_v2.JS:

var mysql = require('mysql');
var md5 = require('md5');
var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'usr',
  password : 'pw',
  database : 'dbname'
});

module.exports = {
    checkUser: function (felhasznalonev, jelszo, callback) {

      connection.connect(function(err) {
        if (err) {
          console.error('error connecting: ' + err.stack);
          return;
        }
      });

      connection.query('SELECT COUNT(*) AS szamlalo FROM felhasznalok WHERE fhsz_email="' +
      felhasznalonev + '" AND fhsz_jelszo= "' + md5(jelszo) +'";', function (error, results, fields) {
        console.log(results[0].szamlalo);
        callback(results[0].szamlalo);
      });

      connection.close();
  },

    getTranzakciok: function () {

      connection.connect(function(err) {
        if (err) {
          console.error('error connecting: ' + err.stack);
          return;
        }
      });

      connection.query('SELECT * FROM tranzakciok";', function (error, results, fields) {
        console.log(results[0]);
      });
  }
}

412-я строка мопса index.js:

 // support callback API
  if ('function' == typeof options) { //412th LINE
    fn = options, options = undefined;
  }
  if (typeof fn === 'function') {
    var res;
    try {
      res = exports.renderFile(path, options);
    } catch (ex) {
      return fn(ex);
    }
    return fn(null, res);
  }

  options = options || {};

  options.filename = path;
  return handleTemplateCache(options)(options);
};

Прошу прощения за мои английские ошибки и благодарю вас за помощь.