Serial field problem with Access 97

I have a PostgreSQL 7.02 database on a Linux RedHat 5.2 server, my
frontend is Access97.

I have linked tables, using ODBC drivers, with serial fields as indexes.

The problem is that every time I try to insert a register Postgres
complaints with the following message:

Insert on a linked table "tablename" failed
Cannot insert a duplicate key into unique index "indexname-pkey"

We don't set the value of the serial index fields on Access, we relay on
Postgres for such task.

Before we move to the RedHat server, we were working on our experimental
server, wich is a Linux Slackware 7.1 box with PostgreSQL 7.02, and
everything worked fine.

I could use some enligthment here. Thanks in advance.