dpm() in hook_init shows twice on the page In my custom module, I use a hook_init() with a dpm-function. 

function mymodule_init() {
    dpm('check');
}


but for some reason, my page shows 2 times 'check' on the page... Is this default Drupal behavior? And if so, what are the reason for this behavior?
hook_init runs once per-page request, for non-cached pages.

See _drupal_bootstrap_full

module_invoke_all('init');


I guess what's perhaps happening in your case is an additional ajax request is bootstrapping Drupal and triggering an additional hook_init call.