For anyone using Yoast SEO and YOOtheme:
I had this very problem using YOOthemes – it looks like my theme company tried to place the blog title code in the header.php (its called head.php on the theme … inside a folder called warp > systems > wordpress > layouts > head.php) – they basically exported their joomla themes to wordpress so now you can customize WordPress like Joomla, its pretty sick. But I wanted to point out that at first I was pulling my hair out trying to blame the Yoast SEO plugin until I found the following code (SPECIFIC TO YOOTHEME):
<title><?php bloginfo('name'); ?> <?php wp_title(); ?></title>
I now change it to the following and it works great:
<title><?php wp_title(); ?></title>
Again, this is specific to YOOtheme but I figured I’d contribute my solution because … well, I absolutely LOVE Yoast’s SEO plugin!!!!
My favorite part about Yoast SEO aside from the customized titles is the ability to remove /category/ from blogs (with the caveat to not have the same category name/slug as a page’s name/slug)