Prevent duplicate values from being entered in a combination of fields

1. Open the table in Design view.
   
   
2. Create a multiple-field index using the fields you want to prohibit duplicate values for. Leave the Indexes window open when you have finished defining the index.

   How?

3. In the upper portion of the Indexes window, click the new index name.
   
   
4. In the lower portion of the Indexes window, click the Unique property box, and then select Yes.