How do I find duplicates across multiple columns?

 SELECT name, city, count(*) as qty 
 FROM stuff 
 GROUP BY name, city HAVING count(*)> 1

Duplicated id for pairs name and city:

select s.id, t.* 
from [stuff] s
join (
    select name, city, count(*) as qty
    from [stuff]
    group by name, city
    having count(*) > 1
) t on s.name = t.name and s.city = t.city