How to prevent multiple selections of the same value in multiple fields

I have a user-input form with a series of questions that have 4 differently ranked answers for each question.  No duplicate ranking is allowed.  Example Question: "My favorite food is:"  Rank these possible answers from 1-4 in order of preference: Apple; Ice Cream: Fish; Wine

I'm using Acrobat Pro DC.  Can I create a script under either Calculate or Validate that would make this work?  If so, what would that be?  If not, any suggestion?

I have created 4 dropdown boxes for each of the 4 form fields.  They are named: Dropdown1a, Dropdown1b, Dropdown1c, Dropdown1d.  I would create/name the remaining fields as Dropdown2a, Dropdown2b, Dropdown2c, Dropdown2d, and so on.

I don't use JavaScript, but here's the simple script that I thought might work:

if(Dropdown1a) not= (Dropdown1b)
if(Dropdown1a) not= (Dropdown1c)
if(Dropdown1a) not= (Dropdown1d)

Any help is much appreciated.  Thank you.