Chain the filters:
if ( 'include' == $query->query_vars['orderby'] )
add_filter( 'posts_orderby', /* … */ );
And in your orderby filter use:
remove_filter( current_filter(), __FUNCTION__ );
Chain the filters:
if ( 'include' == $query->query_vars['orderby'] )
add_filter( 'posts_orderby', /* … */ );
And in your orderby filter use:
remove_filter( current_filter(), __FUNCTION__ );