I was unaware there was more than one pair. I’ve seen the pair in the Smithsonian. I thought they were the only pair.
I knew there was more than one pair, and it makes sense that this was the case.
If there was only one pair and they were damaged during the filming of the movie while Judy Garland was wearing them, production would have had to stop while they were being repaired, and that stop in filming would have cost MGM both time and money. Hence the need for at least one backup pair of slippers.
I’m guessing that there a few other pairs of slippers that were used in the close up scenes; perhaps those pairs had to be more detailed than the other pairs of slippers.