DETACH PARTITION vs merges = duplicates

test case:
```
create table test2 Engine=MergeTree ORDER BY number AS SELECT number, toString(rand()) x from numbers(100000000);
alter table test2 detach partition tuple();
detach table test2; attach table test2; -- or just restart clickhouse
alter table test2 attach partition tuple();
select count() from test2;
``` 

Expected 100000000, got 200000000
