Maybe this is an issue with your php settings regarding output buffering?

http://php.net/manual/de/function.headers-sent.php#75835

Because normally the headers will not be send again:

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/Response.php#L369

dmaicher, thank you for response.

Output buffering allows you to send header lines (including cookies) even
; after you send body content, at the price of slowing PHP's output layer a
; bit.

This is not that case, because headers are sent before the body content.
headers_sent function returns true, only when headers were sent, for instance when I do "echo 'text';". Before I output anything, it will return false. (I have output_buffering=Off in php.ini).
In my case, what is happening:

1. header('Content-Type: text/html; charset='UTF-8');
2. headers_sent(); // returns false
3. header('Content-Type: text/html; charset='UTF-8');
4. echo 'some error';

As a result, header Content-Type is duplicated, I get FCGI error.

Ok indeed for me in your example headers_sent() also returns false 😋
But I don't get any error using php-fpm & nginx.

i've experienced this very same issue a few times as well, mostly triggered by the ExceptionHandler.
I've dug a bit in there and found the following problem:

1. ExceptionHandler puts a Content-Type header into the "header list" to be sent
2. HttpFoundation Response uses the headers_sent method to check if any headers have been sent, this returns false if you use fcgi, because the headers have not been sent, only "added" to the "header list"
3. HttpFoundation Response adds it's headers to the "header list"
4. fcgi tries to send all headers in the "header list" and encounters a problem, because Content-Type was added twice

I've also found a possible solution, instead of using headers_sent to check in the sendHeaders method, !empty(headers_list) could be used to check, if headers have been "scheduled" to be sent.

Example:

<?php
public function sendHeaders()
{
        // headers have already been sent by the developer
        if (headers_sent() || !empty(headers_list())) {
            return $this;
        }
...
}

i could open a pull-request for this, if no-one sees any obvious fault with this?

Thanks Mneuhaus, your patch works !

Arf... now I have a mistake every morning at the same time. this error is triggered by googlebot every morning at 2:20 AM, every day.

[Thu Dec 07 02:20:34 2017] [error] [client 66.249.64.19] [host www.binano.fr] (104)Connection reset by peer: AH10143: FastCGI: comm with server "/homez.xx/user/www/viewtopic.php" aborted: read failed
[Thu Dec 07 02:20:34 2017] [error] [client 66.249.64.19] [host www.binano.fr] AH10149: FastCGI: incomplete headers (0 bytes) received from server "/homez.xx/user/www/viewtopic.php"
[Thu Dec 07 02:20:34 2017] [error] [client 66.249.64.19] [host www.binano.fr] AH10157: FastCGI: An error happend on Fastcgi processing, fallback to CGI

@mneuhaus Is there any existing PR for this fix?

Currently it isn't possible to use sendHeaders with output buffering on, without having multiple headers sent.

@thejahweh no, there is no PR yet, because i can't really tell the implications, because this is quite deep in the core of Symfony. i could open one of course.

I'm having the same issue with Symfony 4.0 and PHP 7.1

I have the same issue. Content-Type is sent 2 times sometimes when an exception is thrown because of ExceptionHandler.

Because of this, I see a 500 internal server error instead of the Symfony Exception page.

By using @mneuhaus fix in HttpFoundation\Response::sendHeaders() I see again the Symfony Exception page.
Or, another way to fix it, is to set the replace parameter of the header function to true.

// headers
foreach ($this->headers->allPreserveCaseWithoutCookies() as $name => $values) {
    foreach ($values as $value) {
        header($name.': '.$value, true, $this->statusCode);
    }
}

I don't have enough knowledge of the HttpFoundation to understand if these are good solutions.
I'm using Symfony 3.4.4 and PHP 7.2.1.

you shouldn't set the replace to true as for example the Links header can appear multiple times and this is perfectly legal.

I was wondering why the ExceptionHandler::handle method triggers the ExceptionHandler::sendPhpResponse who sets the header "Content-Type" when there is a Response object responsible of putting that header in the first place? It seems a nonsense to me, what am I missing?

Any news on this error ? I started few weeks, having the same kind of errors on production website..

I understand what happens here. Looking for a fix now.

Introduced in #10941. Headers need to be recalled like the body is already.

Should be fixed by #29057, please confirm if possible.

Should be fixed by #29057, please confirm if possible.

I was curious about how to fix this so I read the fix you propose and it might solve the issue in most cases but actually the last thing setting "Content-Type" header will be used no matters what. And my question remains unanswered, are we sure ExceptionHandler::handle is really meant to set this header?

I do not know if I made my point clear I can try to be more exhaustive if you want :)

The answer is yes: the exception handler buffers a response that should be sent to the browser whenever the regular error handling fails. When it doesn't, it's the responsibility of the regular error handling to take over.