Error on deleting

Hi:

I have a PostgreSQL 7.2.2 database since 2 years working right until now.
When I try to delete a row from pgAdmin II , I get the message " ....cannot
uniquely identify, 2 rows match....." If I go on deleting the row, the 2
ones are deleted. Of Course the 2 rows don't match. They have different
primary key, OIDs, and so on.

This has happened several times in the last days. If I issue the SQL command
"Delete ......" no errors occurs.

Please I need help, because the database is into production, and I am
receiving callings from angry users which can not delete rows from their
Visual Basic program ( they access the PostgreSQL database via this
program, and get the message " the record does not exist" when they are
trying to delete).

Thanks in advance, and sorry for my poor english.

Indibil