SQL & PL/SQL

Hi,

I have a table it's called loadsurvey which having some duplicate records. I would like to implement unique constraint on my table using following command which should not allow any new duplicate record into table and do not wants to validate existing record.

ALTER TABLE LOADSURVEY ADD ENABLE NOVALIDATE
CONSTRAINT LOAD_UNIQUE_CONS UNIQUE (METERNO,DAT,TIM);

SQL COMMAND WINDOWS SHOWS "ORA-01735: INVALID ALTER TABLE OPTION"

Any help me to come out the problem.

Regards,
Saravanan.