yes i agree we can do it by identity column. But i can not change the column constraint(i.e can not alter the column or add constraint) in production.

tbl2 is...

Actually tbl2 may have duplicate row. But while selecting itself i want to avoid the duplicate data. It's like below

create table tbl(f int)

insert into tbl value(1)

insert into tbl value(1)

insert...