If a plugin is written well (or the theme) than you have no reason to worry about duplicate jquery loading.
if your theme uses header.php to load jquery then your doing it wrong and should use enqueue scripts to load jQuery...
Wordpress uses wp enqueue script to declare scripts once and not multiple times.
Example of loading scripts the right way:
wp_enqueue_script( 'jquery' );
wp_enqueue_script( 'bootstrap', 'http://netdna.bootstrapcdn.com/bootstrap/3.1.1/js/bootstrap.min.js', array('jquery'), 3.3, true);
wp_enqueue_script( 'miscscripts', get_template_directory_uri().'/js/misc-scripts.js', array('jquery'), 1.0, true);
add_action( 'wp_enqueue_scripts', 'sg_theme_js' );
i have downloaded the plugin and found where scripts are being loaded (in testimonial_1.php) and the plugin doesnt load jquery - i guess the author assumes your theme includes jquery correctly (as it should)
i would search for the spot you include jquery in your theme and change the way to do that to the right way AKA using the wp_enqueue_script function