[hibernate-dev] Unique key constraint

Hi, I already have an id as the primary key for the table say XYZ. Now, I want to set a field or a combination of two fields as a unique key so that when i try to save say 2 entries in the DB with same value for the unique key field then only one entry should be made in the table with one primary key. I want hibernate to handle it internally and not throw me an exception. Now i have only one auto-generated primary key and when i try to save 2 entries in the DB with same value for the unique key field it stores duplicate entries. Is it possible? If yes how? Thanks in advance Ankita.