thehotpeg

Hi,

=> Disable ‘Force Rewrite’ option.
=> Create a functions.php in the Child Theme’s Folder

<?php

function my_remove_title_filter(){
   remove_filter( 'wp_title', 'your_themes_filter_wp_title' );
}
add_action( 'init', 'my_remove_title_filter' );

?>

=>Please paste ‘Your_theme’s_filter_title’
This will probably resolve the issue.
I found this info from following sources..
For details http://venutip.com/content/right-way-override-theme-functions and http://www.prasannasp.net/fix-seo-by-yoast-prints-site-title-twice/