odbc -> oracle allowing duplicate entries?

Hi

I am using Access as a front-end to an Oracle database so have
therefore got linked tables using ODBC.

One of my data tables has a joint primary key....i.e. 2 primary keys
(siteID, Month).

I would have expected oracle to complain when trying to add an
identical record with the same siteID and month but it allows it?

If I have a table with one primary key, as I do on another table,
oracle won't let me add a duplicate record...as I would expect.

The design view in Access shows 2 primary keys. Why is Oracle happy to
let me add an identical row. Is this an ODBC issue and Access?

I want to get users to upload data through an excel spread*** and
can't have duplicate records being accepted....

any ideas?
bib

.