Drop Down List Should Exclude Previously Selected List Items
I am trying to limit the options in a dropdown list so that a user cannot select "option A" for both "slot 1" and "slot 2". In the attached image, the equivalent would be blocking the user from assigning "testtest" or "matt 622 upload" in the third slot for the parent "CUMMINGS, JAMIN".
Note: every parent should be able to select every option once, but never more than once. What is the easiest way to do this? Is there a certain way I need to setup my lists?
Thank you in advance for any help.