Connected to Oracle Database 10g Express Edition Release 10.2.0.1.0
Connected as hr
SQL> drop table t;

Table dropped

SQL> create table t( i timestamp);

Table created

SQL> alter table t add constraint uk_i unique(i);

Table altered

SQL> BEGIN
  2    FOR i IN 1 .. 1000 LOOP
  3      INSERT INTO t VALUES (systimestamp);
  4      COMMIT;
  5    END LOOP;
  6  END;
  7  /

BEGIN
  FOR i IN 1 .. 1000 LOOP
    INSERT INTO t VALUES (systimestamp);
    COMMIT;
  END LOOP;
END;

ORA-00001: unique constraint (HR.UK_I) violated
ORA-06512: at line 3

SQL> SELECT * FROM t;

I
----------------------------
10/03/2007 14:04:51,136000

SQL>