How do I use the Simple HTML DOM Parser in plugin when other plugin already uses it?

If you don’t want to mess with plugin load order, you can trigger the require part a bit later, e.g. via the plugins_loaded hook.

function csp_require_simplehtml() {
    if (!function_exists('file_get_html')) {
        require(CPS_PLUGIN_PATH . '/vendor/simple_html_dom.php');
    }
}
add_action('plugins_loaded', 'csp_require_simplehtml');