Prevent duplicate records in a query based on fields in the underlying table

1. Open a query in Design view.
   
   
2. Select the query by clicking anywhere in query Design view outside the design grid and the field lists.
   
   
3. Click Properties on the toolbar to display the query's property sheet.
   
   
4. Set the UniqueRecords property to Yes.

Notes  

• The UniqueRecords property has an effect only when you use more than one table in the query and select fields from those tables.
  
  
• When the UniqueRecords property is set to Yes, Microsoft Access automatically sets the UniqueValues property to No.
  
  
• As an alternative to setting the UniqueRecords property, you can add to the design grid the field that distinguishes one record from another.