Web site browser title is duplicated

Your site seems to be running fine ? Have you updated your browsers. Also clear the browser cache and cookies.

look into header.php and see what code is used for

<title> ???? <?title>

this possibly should only be:

<title><?php wp_title(); ?><?title>

as you are doing the seo with a plugin <!-- This site is optimized with the Yoast WordPress SEO plugin v1.3.4.4 - https://yoast.com/wordpress/seo/ -->

Yes, an astute observation, when I activated the Yoast SEO plugin this problem started, it seems.
Now, here’s the code from header.php, and as I’m not a code person, it seems considerably more complicated that yours above:

<title>
            <?php
            /*
             * Print the <title> tag based on what is being viewed.
             */
            global $page, $paged;
            wp_title('|', true, 'right');
            // Add the blog name.
            bloginfo('name');
            // Add the blog description for the home/front page.
            $site_description = get_bloginfo('description', 'display');
            if ($site_description && ( is_home() || is_front_page() ))
                echo " | $site_description";
            // Add a page number if necessary:
            if ($paged >= 2 || $page >= 2)
                echo ' | ' . sprintf('Page %s', max($paged, $page));
            ?>
        </title>

Do I replace with your simpler version?

Do I replace with your simpler version?

you can try it …