Duplicated Title-Tag

Hello,
I’m using an Omega child theme on my site and since the last WP releases it looks like the title-tag is generated twice : once by the theme, and another time by the SEO Plugin (Yoast). Formerly I got rid of this trouble by checking “force rewrite title” in the seo plugin settings but now this field is even not displayed anymore.
I’d like to keep the SEO title, so how could I disable the title-tag generated by the theme itself?
Any piece of code to add to my child theme functions.php? Anything else?
Thanks a lot for the help.