Magento custom controller action is called twice - php

Question

I added a ajax add to cart to the rwd theme, and the action controller is called twice for every http request. Any help with solving the problem or debugging is welcome, I already lost 2 weeks on this. This all works on our development environment, but acts weird on staging. The dev environment is hosted locally on MAMP, staging is hosted on OVH shared hosting.
edit2: removed irrelevant info

Answer 1

found this error message in error.log
FastCGI: comm with server "/..../staging/index.php" aborted: error parsing headers: duplicate header 'Content-Type', referer: http://staging.xxxx.be/product.html
also found a solution here: http://blog.imseo.it/2014/09/08/magento-fastcgi-error-parsing-headers-duplicate-header/
The solution is to replace the function sendHeaders in
app/code/core/Mage/Core/Controller/Response/Http.php
public function sendHeaders()
{
if (!$this->canSendHeaders()) {
Mage::log('HEADERS ALREADY SENT: '.mageDebugBacktrace(true, true, true));
return $this;
}
if (in_array(substr(php_sapi_name(), 0, 3), array('cgi', 'fpm')))
{
// remove duplicate headers
$remove = array('status', 'content-type');
// already sent headers
$sent = array();
foreach (headers_list() as $header)
{
// parse name
if (!$pos = strpos($header, ':'))
continue;
$sent[strtolower(substr($header, 0, $pos))] = true;
}
// raw headers
$headersRaw = array();
foreach ($this->_headersRaw as $i=>$header)
{
// parse name
if (!$pos = strpos($header, ':'))
continue;
$name = strtolower(substr($header, 0, $pos));
if (in_array($name, $remove))
{
// check sent headers
if ($sent[$name])
{
unset($this->_headersRaw[$i]);
continue;
}
// check header
if (!is_null($existing = $headers[$name]))
{
$this->_headersRaw[$existing] = $header;
unset($this->_headersRaw[$i]);
}
else
$headersRaw[$name] = $i;
}
}
// object headers
$headers = array();
foreach ($this->_headers as $i=>$header)
{
$name = strtolower($header['name']);
if (in_array($name, $remove))
{
// check sent headers
if ($sent[$name])
{
unset($this->_headers[$i]);
continue;
}
// check header
if (!is_null($existing = $headers[$name]))
{
$this->_headers[$existing] = $header;
unset($this->_headers[$i]);
}
else
$headers[$name] = $i;
// check raw headers
if (!is_null($existing = $headersRaw[$name]))
unset($this->_headersRaw[$existing]);
}
}
}
parent::sendHeaders();
}