add login error when user already logged in

index.js

@@ -104,7 +104,9 @@ app.post('/signup', (req, res) => {
 app.post('/login', (req, res) => {
   if (!req.body.username || !req.body.password) {
     return res.redirect('/signup');
-  }
+  } else if (req.session.user) {
+    error(req, res, 'User already logged in', 'You must logout before log in.')
+  } else {
     User.findOne({ username: req.body.username }, (err, user) => {
       err ? error(req, res, 'Error fetching user', err) : null;
       if (bcrypt.compareSync(req.body.password, user.passwordHash)) {
@@ -115,6 +117,7 @@ app.post('/login', (req, res) => {
         return res.redirect('/signup');
       }
     });
+  }
 });
 app.post('/logout', (req, res) => {
   req.session.destroy();