Resolved with solution from @fayewatson:
"I would create a new data type called Vendor, and a field within that data type called “Name” (to store the name of the Vendor). In the New Order data type, the Vendor field would be “Name of Field: Vendor, Type: Vendor, List: No” instead of “Name of Field: Vendor, Type: Text, List: No”.
Then in your dropdown, you can have the data type be Vendor, choices Sources be: Search for New Order’s Vendor, and Option Caption: Current Option’s Name.
I just updated the forum app example with the new fields. If you look at the New Order data type, the Vendor field now contains the the unique ID of the associated Vendor.
If you click ‘Primary fields’, you can change the Vendor data type view to display the Vendor name instead:
Which changes it to this:
It looks the same as when you had it, but it’s actually removing the duplicate data problem from before. If you go to the Vendor data type you’ll see no duplicates:
Now the dropdown will only display all of the Vendors connected to the New Orders."