Thank you very much for the response, I feel vindicated as I have a PhD and my friend has a Master's, and I was pretty angry that he seemed to completely ignore my greater experience in academia but I didn't want to start pulling rank as that would have just poured petrol on the fire.

Please could you elaborate on your thought process in the hypothetical situation I described, where a PhD graduate would apply for a second PhD (or third etc.). What specifically may be your reasons for being punitive of their application? Would there be practical reasons, such as it would be inefficient to train a person on your research area if they are not going to have any input into it beyond their PhD? Or would it be personal, like "why should I invest myself in someone who will not make me proud"? Or perhaps just purely... out of spite(!), i.e. "I'm not gonna help this guy build a huge ego for himself"? Perhaps a combination of reasons?