Maybe you should clarify that we have to enter the name of each Caliph individually (i.e. with their numbers) even if they have the same name. For such quizzes, entering the name "Abd ar-Rahman" once would be enough to get all the duplicate Abd ar-Rahman(s). You follow this on your "All Caliphs" quiz, so no reason to make it more difficult here, even if the result is a rather simple quiz. It's not our fault they all repeated their predecessor's names, but it's usually like this in monarchies.
Thanks for the feedback. I've thought about this quite a bit and I'm still not sure what the best way is. I will reconsider doing it the way you describe again.