Need to prevent duplicate and "mirror" pairings being selected in DropLists

Attached shows a Range of models, and the dependent Ranges of models to which each one can be compared.

User has three boxes where he selects a model in the DropList, then selects from the Dependent DropList the Model against which it will be compared.

Problem 1: If user selects "A vs B" in a box, then selects "A" in another box, how do I prevent "B" from being selected as the comparison again?

Problem 2: If user selects "A" v B" in one box, how do I prevent him from selecting the reverse ("B v A") in another box?

Any suggestions, pointers or solutions accepted gratefully

Ochimus