Node.js: http.js:691 throw new Error('Can\'t set Kopfzeilen, nachdem Sie gesendet werden.')

Hallo, ich mache einfache Anwendung mit MEAN-stack. Und ich bekomme diese Fehlermeldung http.js:691 throw new Error('Can\'t set Kopfzeilen, nachdem Sie gesendet werden.')

Hier ist der code:
server.js

//set up
var express = require('express');
var app = express();
var mongoose = require('mongoose');

//mongodb connection and app configuration
mongoose.connect('mongodb://localhost/test');

app.configure(function () {
    app.use(express.static(__dirname + '/app'));
    app.use(express.logger('dev'));
    app.use(express.bodyParser());
    app.use(express.methodOverride());
});

//define model
var Employee = mongoose.model('Employee', {
    firstName : String,
    lastName : String,
    photoUrl : String
});

//routes ==========================
//api
//GET
app.get('/api/employees', function(req, res) {
    console.log("GET");
    Employee.find(function(err, employees) {
        if(err) {
            res.send(err);
        }

        res.json(employees);
    });
});

//Get by ID
app.get('/api/employees/:employeeId', function(req, res) {
    Employee.findById(req.params.employeeId, function(err, employee) {
        if(err) {
            res.send(err);
        }

        res.json(employee);
    });
    Employee.find(function(err, employees) {
        if(err) {
            res.send(err);
        }

        res.json(employees);
    });
});

//POST
app.post('/api/employees', function(req, res) {
    Employee.create({
        firstName : req.body.firstName,
        lastName : req.body.lastName,
        photoUrl : req.body.photoUrl
    }, function(err, employee) {
        if(err) {
            res.send(err);
        }
        Employee.find(function(err, employees){
            if(err) {
                res.send(err);
            }
            res.json(employees);
        });
    });
});

//DELETE
app.delete('/api/employees/:employee_id', function(req, res) {
    Employee.remove({_id : req.params.employee_id}, function (err, employee) {
        if(err) {
            res.send(err);
        }

        Employee.find(function(err, employees) {
            if(err) {
                res.send(err);
            }
            res.json(employees);
        })
    });
});

//application
app.get('*', function(req, res) {
    res.sendfile('./app/index.html'); 
});

//start listening (start app)
app.listen(8080);
console.log("App listening on port 8080");

Und auch:
app.js Winkel-Routen

var employeesApp = angular.module('employeesApp', [
    'ngRoute',
    'EmployeesControllers',
    'employeeAppAnimations'
]);

employeesApp.config(['$routeProvider',
    function($routeProvider) {
        $routeProvider.
            when('/employees', {
                templateUrl: 'partials/employees_list.html',
                controller: 'EmployeesCtrl'
            }).
            when('/employees/:employeeId', {
                templateUrl: 'partials/employees_details.html',
                controller: 'EmployeeDetailsCtrl'
            }).
            otherwise({
                redirectTo: '/employees'
            });
    }]);