OS backup duplicate names

[Irl]

I use SD! to make backups of my Catalina (10.15.7) OS drive; they are named "MB2OSXBkp1" and "MB2OSXBkp2". Both backups are volumes within an APFS container on an external HD. When I restart and hold the option key, the "choose start volume" screen shows four drives, named "MB2OSXBkp1", "MB2OSXBkp2", "MB2OSXBkp1", and "MB2OSXBkp2". If I choose either of the first two, boot fails, with the big "Ø", followed by boot to the normal drive. If I choose either of the second two, boot proceeds normally and the system volume ends up being as expected. Why are there duplicate drive names, and how can I get rid of them? I recently upgraded to Catalina.