The last couple of days I have been dealing with an irritating problem that especially manifested itself on new WordPress installs, not on existing ones.
Once activated WordPress SEO would show double site titles no matter what settings I was using. I never use the force rewrite function and instead adjust the code in my header.php file into
<title><?php wp_title( ' ' ); ?></title>
I have solved this highly irritating problem though and I'd thought I share it here as it seems that many people here have the exact same problem.
I first deleted the entire plugin and installed an older version of it instead. To do that, you can go to "Other Versions" in the sidebar on the main page of the plugin.
I took version 1.1.2, but you can basically take anything recent.
I installed this older version and had a look whether my titles were showing as I intended to, nothing however. Then I auto-updated the plugin by clicking the update link on my install's plugins page, went to my permalinks and saved them without touching or changing anything and... magically my title appeared correctly!
I am not 100% sure whether installing an older version has anything to do with it or if it even helps. It might be a combination of a few of the things I tried.
Maybe you'd need to deactivate and delete the plugin, reinstall the thing and visit your permalinks, who knows?
Anyways, it's working for me now, hopefully it will for you soon too!