Preventing Dups for 2 fields

[PK]

I think the answer to this is simple, yet it keeps eluding me!

I have two fields in a table DEPARTMENT, and ROLE.

I need to prevent duplicate DEPARTMENT-ROLE entries.

For Example:

Rec DEPT ROLE
1 Payroll Clerk
2 Accounting Clerk <<< ENTRY WOULD BE OK >>
3 Payroll Clerk <<<NEED TO PREVENT THIS ENTRY>>>>

Thanks in advance for any help!