weird duplicate data problem

a table was created with:

create table bills (
id serial not null,
account_no int4 not null,
date_issued date not null,
.....
);

The following SQL script returns only 1 row of data:

select oid, * from bills where id = xxxxx' order by oid;

However with the following SQL:

select oid, * from bills where date_issued = '01/01/2001'
order by oid;

some of the data is duplicated, including the OID.

When trying to 'vaccum' the database, get

NOTICE: Index bills_id_key: number of index tupples (4755) is not the
same as heap (7800).

What is happening? What could have caused this?

Database version used is V6.50. I know it's old, but i hope it's not the
version that's causing this problem.

Thanks in advance for any suggestions.