I have tested it with 100 calls to the esi view helper, and the header is send once. It is added to the `$headers` array in (https://github.com/zendframework/zf2/blob/master/library/Zend/Http/Headers.php) though, and there is some preg_matching happening. All that extra calls are simple to prevent by adding a boolean in de Esi view helper indicating if the surrogate-control header is already added. What do you think?