WordPress page title repeated in SOME pages

In order to play nicely with Plugins or other code that attempts to modify the HTML document title content (i.e. wp_title() output), you should always and only output this:

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

…and if you want to modify that output yourself, filter wp_title instead of hard-coding anything inside the <title></title> tags.

So for your example code:

<title><?php wp_title(''); ?><?php if(wp_title('', false)) { echo ' | '; } ?><?php bloginfo('name'); ?></title>

Replace all that with:

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

…and then add a callback to the wp_title filter:

function wpse72446_filter_wp_title( $title ) {
    if ( '' == $title ) {
        return get_bloginfo( 'name' );
    }
    return $title;
}
add_filter( 'wp_title', 'wpse72446_filter_wp_title' );

That way, your modifications will play nicely with any Plugin _doing_it_right() (like Yoast SEO).