Problem when adding an existing primary key

Hi, I'm new to Postgres and I'm using it for 1 month now.

The problem is:

When I try to insert via application a registry that already exists
Postgres shows an error  "Unable to insert duplicate primary key on index '=
index' ".
I think it's an error, because I would treat this error, just like others.=
=20

I've found this message in file: nbtinsert.c line 257.

I'm using Postgres 7.2 but I've seen this message in version
7.3.1.

Thanks in advance,
Ricardo.