I'm new to WordPress SEO but just got rid of that problem by going to SEO > Titles & Metas in the dashboard. For what it's worth, I checked the first box on that page:
Force rewrite titles
WordPress SEO has auto-detected whether it needs to force rewrite the titles for your pages, if you think it's wrong and you know what you're doing, you can change the setting here.
When I did that, my double titles disappeared.