Model a Unique Constraint
Unique constraints are a combination of values (represented by Table columns) that uniquely identify a row in the Table.
Access Tools | Database Builder | Click on Table name > Constraints | Right-click | Add New Constraint (Ctrl+N)
A new constraint is added to the Constraints tab, with the default type index. Click on the drop-down arrow and select unique. See the Help topic Unique Constraints for information on defining the new constraint.
To delete a unique constraint you no longer require, right-click on the constraint name on the Constraints tab and select the Delete constraint <name> option. The constraint is immediately removed.