"Can't set headers after they are sent." example

example.js

  app.post('/recover_password', function(req, res) {
    User.get(req.body.username, function(err,data) {
      if (err) {
        req.session.error = 'We could not find this user.';
        res.redirect('back'); // ERROR THROWN HERE
      } else {
        var user = new User(data);
        user.resetPassword(function(err,data) {
          var newUser = new User(data);
          newUser.sendPasswordRecoveryEmail();
        });
        res.redirect('/recover_password_confirmation');
      }
    });
  });

How did you solve this? I'm having the exact same problem.