app.user.username in twig returns unable to add global - authentication

Question

as per the documentation:
http://symfony.com/doc/current/book/security.html#retrieving-the-user-object
I am trying to use twig to check to see if the user is authenticated
when i put in my twig templates:
{{ app.user.username }}
I get the error
LogicException: Unable to add global "app" as the runtime or the extensions have already been initialized.
EDIT: Please find as per below the composer.json contents
{
"name": "symfony/framework-standard-edition",
"description": "The \"Symfony Standard Edition\" distribution",
"autoload": {
"psr-0": { "": "src/" }
},
"require": {
"php": ">=5.3.3",
"symfony/symfony": "2.1.*",
"doctrine/orm": ">=2.2.3,<2.4-dev",
"doctrine/doctrine-bundle": "1.0.*",
"twig/extensions": "1.0.*#dev",
"symfony/assetic-bundle": "2.1.*",
"symfony/swiftmailer-bundle": "2.1.*",
"symfony/monolog-bundle": "2.1.*",
"sensio/distribution-bundle": "2.1.*",
"sensio/framework-extra-bundle": "2.1.*",
"sensio/generator-bundle": "2.1.*",
"jms/security-extra-bundle": "1.2.*",
"jms/di-extra-bundle": "1.1.*",
"kriswallsmith/assetic": "1.1.*#dev",
"doctrine/data-fixtures": "dev-master",
"doctrine/doctrine-fixtures-bundle": "dev-master"
},
"scripts": {
"post-install-cmd": [
"Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::buildBootstrap",
"Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::clearCache",
"Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets",
"Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installRequirementsFile"
],
"post-update-cmd": [
"Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::buildBootstrap",
"Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::clearCache",
"Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets",
"Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installRequirementsFile"
]
},
"extra": {
"symfony-app-dir": "app",
"symfony-web-dir": "web"
}
}

Answer 1

Its hard to answer to this question without more details.
First, please notice that there have been such a similar exception in the past, due to a Symfony2.1 bug which has been solved with a recent release: see here for more details.
Then, notice that the error seems to be thrown in addGlobal function of class Twig_Environment. Did you installed such Twig Extension?
Without reading your composer.json, I can suggest to try to:
Clear the cache.
update the code in your composer.json, to avoid the presence of up to date bundles.
Create the composer.phar.
Then compile it to get the composer.phar.
php composer.phar install or php composer.phar update, whatever be your case.
Hope it helps.