TYPO3 Core

Headers with the same name are always overridden when outputting a response in extbase.

// TYPO3\CMS\Extbase\Core\Bootstrap
foreach ($response->getHeaders() as $name => $values) {
    foreach ($values as $value) {
        header(sprintf('%s: %s', $name, $value));
    }
}
// https://www.php-fig.org/psr/psr-7/
// Emit headers iteratively:
foreach ($message->getHeaders() as $name => $values) {
    foreach ($values as $value) {
        header(sprintf('%s: %s', $name, $value), false);
    }
}