create table t (id int, b int, c int, primary key(id) clustered);
insert into t values (1,1,1);

-- during backfill
delete from t where id = 1;
insert into t values (2,1,1);
delete from t where id = 2;

-- before merge
insert into t values (3, 1, 1);

alter table t add unique index b(b), add index c(c);
admin check table t;