How to delete duplicate entries from listbox before display?

Hi there.

I have a view called: used cars.
When using the form to add an entry to this.
There is a field called: make
which does a lookup on a view called car types.

However.
car types has 2 fields.
make and model.

so if there is 2 records like so:
BMW Z3
BMW 320i.

then the lookup returns 2 * BMW to my drop down list.
How can I remove these duplicates?

Thanks.
Rats.