Does an index where the last column is unique and already indexed make sense?
I can think about one scenario where such index is needed. If you want another table to have a Foreign Key constraint that refers (RetailerID, ID), you must create a unique constraint (index).  This pattern makes sense in some cases, for instance if you want to have a common parent table which holds common attributes, and multiple detail tables, and at the same time enforce that details are saved in the proper table.  

Update. Example to illustrate an idea  : 

vehicle (vehicle_id, vehicle_type_id, year, model, ...
         PK (vehicle_id),  
         Unique (vehicle_type_id,car_id),
         CHECK (vehicle_type_id IN ('car','truck'))
);  

car (vehicle_id, vehicle_type_id default 'car', 
     num_passengers, car_type, ....
     PK,FK(vehicle_id, vehicle_type_id) ref vehicle, 
     CHECK (vehicle_type_id ='car')
); 

truck (vehicle_id, vehicle_type_id default 'truck',  load, 
     dimensions, ... ,  
     PK,FK(vehicle_id, vehicle_type_id) ref vehicle, 
     CHECK (vehicle_type_id ='truck')
);


Thus you enforce that truck's attributes are always stored in truck table, car's - in car, and at the same time refer to vehicle if vehicle type is not important, say vehicle_owner  can look like vehicle_id, person_id, date_from ,date_thru, FK (vehicle_Id) ref vehicle(vehicle_id)).