CREATE TABLE fly_hash_block(tth_id integer PRIMARY KEY NOT NULL, tth number NOT NULL);
INSERT INTO fly_hash_block VALUES(1,1);
INSERT INTO fly_hash_block VALUES(2,2);
INSERT INTO fly_hash_block VALUES(3,2);
CREATE UNIQUE INDEX iu_fly_hash_block_tth ON fly_hash_block(tth);
select * from fly_hash_block;
1|1
2|2
3|2
select distinct tth from fly_hash_block;
1
2
2
select tth,count(*) from fly_hash_block group by tth;
1|1
2|2

SQL> 
SQL> CREATE TABLE fly_hash_block(tth_id integer PRIMARY KEY NOT NULL, tth number NOT NULL);
Table created
SQL> INSERT INTO fly_hash_block VALUES(1,1);
1 row inserted
SQL> INSERT INTO fly_hash_block VALUES(2,2);
1 row inserted
SQL> INSERT INTO fly_hash_block VALUES(3,2);
1 row inserted
SQL> CREATE UNIQUE INDEX iu_fly_hash_block_tth ON fly_hash_block(tth);
CREATE UNIQUE INDEX iu_fly_hash_block_tth ON fly_hash_block(tth)
ORA-01452: CREATE UNIQUE INDEX невозможно; найдены дублирующиеся ключи
SQL> select * from fly_hash_block;
                                 TTH_ID        TTH
--------------------------------------- ----------
                                      1          1
                                      2          2
                                      3          2
SQL> select distinct tth from fly_hash_block;
       TTH
----------
         1
         2
SQL> select tth,count(*) from fly_hash_block group by tth;
       TTH   COUNT(*)
---------- ----------
         1          1
         2          2