Express finalhandler cannot 404 after headers sent - node.js

Question

I have an express app and i have added new route:
this is the route:
var router = require('express').Router();
router.post('/',function (req, res, next) {
res.send("Ok");
});
module.exports = router;
now on every request that i make to this post route im getting at the express log:
finalhandler cannot 404 after headers sent
When making calls to DB the finalhandler does send 404 for every requst,
so im guessing that there is some kind of race with my functions and the finalhandler
anyone have anyidea?
UPDATE:
This is the index.js
var express = require('express');
var app = express();
var bodyParser = require('body-parser');
var expressValidator = require('express-validator');
var _ = require('underscore');
var stringUtils = require("underscore.string");
var mongoose = require('mongoose');
mongoose.connect(config.mongoUrl);
var db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
app.use(expressValidator());
var middleware = require('./middleware/authentication-middleware');
app.use(middleware.allowCrossDomains);
app.all('*', loginMiddlewareSkip);
var skipAuthPaths = ['/auth/fb', '/auth/login', '/auth/signup', '/auth/forgot', '/auth/reset'];
function loginMiddlewareSkip(req, res, next) {
if (stringUtils.startsWith(req.path, "/auth") || req.path == '/status'){
return next();
}
middleware.ensureAuthenticated(req,res,next);
next();
}
app.use('/passport', require('./routes/passport'));
app.use('/auth', require('./routes/authenticate'));
app.listen(3000, function() {
console.log('Express server listening on port ' + 3000);
});
The file above is the routes/passport, and the auth file is working fine

Answer 1

Ok the problem was with one of my middle wares:
in my loginMiddlewareSkip function i was calling
middleware.ensureAuthenticated(req,res,next) if the path need an authentication, inside that function i was calling next() if authentication is successfull and as you can see i was calling next() again after calling the ensureAuthenticated function.
removing the next() from loginMiddlewareSkip solved the problem