Table-index: duplicate values at runtime

By definition the word "Index" means UNIQUE.

You must construct a Complex Key where the combined values always represent a UNIQUE combination. If you have existing data just open the table in design view and click on the fields you need to participate in the kay and press the "key" button on the toolbar. Press [Shift] to select multiple fields before pressing the "key" button.

If you are starting from scratch select multiple fields the same way AFTER they have been defined and enter a bunch of stuff to make sure it works as expected rejecting combination duplicates but accepting single field duplicates.