Getting Duplicates in drop down menu using row label

[Clifford]

I have a table (CITY) of cities with multiple venues per city. I would like to choose a city then have a dependent dropdown with the venue. I currently have a separate column in the CITY table as a unique key and using row label to have the CITY names in the dropdown referenced from the main table. This is so users can add new venues or I would just use an easy VALID_IF reference.
It seems I could use the UNIQUE(CITY[CITY]) but cannot figure out where to put it??

Any help would be appreciated, THANKS!