Duplicate data entry in Access Database

I’m having a problem controlling data entry in one of my tables. The table tracks software loaded on PCs. This table has 3 feilds, the 1st is an autonumber key field, the second is the client and the third is the software package.

Each time a client gets software, there will be a new record with the clients name and the software name, but each client can only have one copy of each software package.

How do I set my form so that it will look at the two fields together to determine if there are duplicates?