No, the MINUS will remove any duplicates in the same way as a UNION. There are several ways of detecting duplicates:

1) Group by..having..

select STAT_TIMESTAMP, STAT_DTO_ID, STAT_ID, count(*)
from STATS
group by STAT_TIMESTAMP, STAT_DTO_ID, STAT_ID
having count(*) > 1

2) Rowid:

select * from
stats s1
where exists
(select 'x' from stats s2
where s1.statdto_id = s2.stat_dto_id
and s1.stat_timestamp= s2.stat_timestamp
and s1.stat_id = s2.stat_id
and s1.rowid <> s2.rowid)

3) Use an EXCEPTIONS ...INTO clause when enabling the constraint.