If multiple custom search blocks are on the page, submit handler is invoked multiple times.

I ran into this because I needed to override the submit handler of a custom search form, but found that I couldn't. Although I was being nice and simply hooking in my own submit handler to override custom_search's submit handler I found that I was being trumped by yet another submit handler. that's because of this code block in custom_search_form_alter

<?php
        // Form attributes
        $form['#attributes']['class'] = array('search-form');
        $form['#submit'][] = 'custom_search_submit';

It could be this and then it will only have the submit handler registered once:

        // Form attributes
        $form['#attributes']['class'] = array('search-form');
        if (!in_array('custom_search_submit', $form['#submit'])) {
          $form['#submit'][] = 'custom_search_submit';
        }