Time machine error: Two of the disks to backup have the same name.

Hi, I am unable to backup to time machine at the moment. I was having issues with my previous backup drive, so purchased a new Seagate 2TB usb expansion drive, but after connecting it I am getting the exact same error message as with the last hard drive.

I have upgraded iMac to 11.1 Big Sur, but can't remember if I had successfully backed up since that upgrade, or if this has happened since the upgrade.

Troubleshooting steps done so far:

First aid on hard three drives, from both logged in, and in recovery mode.

Creating a partition on a hard drive, called backup, while in recovery mode.

Disk is APFS case sensitive formatted, but tried encrypted and plain APFS to so see if that made any difference. Unfortunately it didn't.

Any assistance on how to get my machine in a position to do a backup would be really appreciated, as I don't like having my data not backed up!