Il y a beaucoup de versions suggérées ici mais je pense en avoir trouvé une nouvelle.
select *
from @T as T1
where exists (select *
from @T as T2
where
T1.ID <> T2.ID and
T1.C1 = T2.C1 and
T1.C2 = T2.C2 and
T1.C3 = T2.C3)
