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<?php
function custom_modify_get_avatar($args, $id_or_email) {
    $avatar_att_id = get_user_meta( get_current_user_id(), '_avatar_att_id', true );
    if($avatar_att_id) {
        $avatar_att_url = wp_get_attachment_image_src($avatar_att_id, array($args['width'], $args['height']));
        if ( !empty($avatar_att_url) ) {
            $args['url'] = $avatar_att_url[0];
        }
    }
    return $args;
}
add_filter('pre_get_avatar_data', 'custom_modify_get_avatar', 10, 2);