Yoast SEO: title tags are repeating my website name and slogan [closed]

wp_title() is filterable so there could be another plugin or most likely your theme already adding a filter. Have tried checking the force rewrite in the plugin options? If that doesn’t work you need to hunt down what else is adding the filter and remove it. If it’s your theme look in functions.php for

add_filter('wp_title', 'some_function_name' );

Remove that line.