So, I made some really significant progress on this and am attaching the result, which is likely to go into the Shared Content section. There is ONE piece I’m still working on, which is the ability to highlight the two duplicated answers instead of just the one that’s been selected.
I was able to get the function to run twice. However, it did not highlight the two duplicates and gave the alert twice (I expected the latter to happen). Any ideas on how I can add the border to the duplicates and only have the alert once?
The sample includes explanations and the originals from @timk for how to use it on regular drop lists and multiple choice questions that use a drop list and all have the same set of possible answers.