select *
    from category
    where category_id IN
        (select cat1 as category_id
            from (select min(category_id) as cat1, max(category_id) as cat2, name, site_id, count(*)
                from category
                group by name, site_id
                having count(*) > 1
            ) as c1
        union    select cat2 as category_id
            from (select min(category_id) as cat1, max(category_id) as cat2, name, site_id, count(*)
                from category
                group by name, site_id
                having count(*) > 1
            ) as c2
        );