If you, like me, developed your WP theme before you activate the excellent WordPress SEO by Yoast you might have double sitenames in the title tag after activating the plugin. Something like:
{Sitename}{Page title} | {Sitename}
In your header.php file, the code of the title tag will look similar too:
<title> <?php echo get_bloginfo('name'); ?><?php wp_title('|', true, 'left'); ?></title>
Here is how I fixed this problem with legacy support in mind:
<title> <?php echo (!function_exists('wpseo_replace_vars')) ? get_bloginfo('name') : ''; ?><?php wp_title('|', true, 'left'); ?></title>
Now the title will always look like you intended.