Do you have a unique constraint, unique index or primary key constraint on BILL_NO ?

If you don't, then there is nothing stopping duplicates from occurring.

If you do, then check to see if the constraint state is: ENABLE / VALIDATED


A common development error is when people write code like:

Step 1) select count(*) from AR_BILL where BILL_NO = 'X';
Step 2) if result = 0 then insert new row

thinking that this will avoid duplicates. It does *NOT* work in a multi-user environment, because the following can happen:

Session 1:
=========
select count(*) from AR_BILL where BILL_NO = 'X';
> result=0
insert new row

Session 2:
=========
select count(*) from AR_BILL where BILL_NO = 'X';
> result=0
insert new row

Session 1:
=========
commit;

Session 2:
=========
commit;

and just like that ... you have a duplicate.