If you want to only show those that are unique, you'll need to base the dropdwon on a Totals query where you have the field twice - once Group By, the other Count. Then add 1 to the criteria of the Count field.
Instead, if you want to show only one of every occurrence in that field (unique or not), go to the Row Source of the combo box. Click the Builder to see the query design, then pick the SQL view. Change SELECT... to SELECT DISTINCT... and you'll see one of each item.
Denis