duplicate key violated errors

Hi All:

I have a table, which has over 3 million rows. We have both a unique
constraint ( which creates implicit index) as well as a regular unique
index on the same column. We had to put the unique constraint because
duplicate values
were getting inserted to the table in spite of the regular unique index
on the column. In our applications we always check to make sure  the
existence of the value before we insert, in another word we do not
insert if the value exist there already.

 Do you know of any known issues where postgres throws duplicate key
violated error in a situation like mine where I have two indexes on the
same column. I know it does sound redundant, but I am skeptical about
changing this any other way since it is a production
system. I did rebuilt the regular index but we still kept on getting the
same error, so index corruption is  out of question.

We run  postgresql 8.2.5

Thanks.
Dinesh