Distinct values from reference table & second reference based on first selection

[jonasjonas]

Hello everyone,

in my App, I am using three tables.

Table name	Key
Cases	case_id
Rentals	rental_id
Vehicles	vehicle_id

Table structure with columns

Cases

Column name	case_id	rental_id	vehicle_id
Description		Reference to Rentals	Reference to Rentals

Rentals

Column name	rental_id	vehicle_id
Description		Reference to Rentals Can be multiple vehicles for each rental_id

Vehicles

Column name	vehicle_id
Description

Note: 
Each rental_id can have multiple lines with different vehicle_id and each vehicle_id can be used in multiple rental_id. 

Questions

1. In the "Cases" form, I want to see a distinct list of "rental_id", even if there is the same "rental_id" in multiple lines. (column = rental_id)
   When I use type=Ref but create a Valid_id, I get distinct "rental_id", but there is a warning sign after every individual "rental_id".
   How can I do that?
2. In the "Cases" form, I want to see a list of  "vehicle_id", which are available for the "rental_id" chosen before (in question 1). (column = vehicle_id) 
   How can I do that?

I want to keep the columns type Ref if possible.

Thanks for your help in advance!