Duplicate Titles & Theme Conflict

We have noted that there are duplicate titles appearing in Google’s search results for some of our pages.
At first we thought this may be an issue with Yoast plugin and we followed various advice about rectifying this and performed the following in functions.php

add_filter('avf_title_tag', 'avia_change_title_tag', 10, 2);
function avia_change_title_tag($title, $wptitle)
{
return $wptitle;
}

This also did not solve the problem. We then changed the title output in header.php to:
<title><?php wp_title(''); ?></title>
This also has not helped and the problem remains. You cannot visually see or find the extra words it keeps adding to the title. But it shows up on some important pages in Google’s search.