Solution for initial Time Machine backup way larger than source

For a few months, my Time Machine backup disk filled up way too fast: I know how incremental backups work, and it made no sense that, starting immediately after the initial backup, the Time Machine volume consistently had almost twice the space of the originating one occupied: with an internal drive of 1 TB capacity, 90% full, my 2 TB Time Machine drive immediately filled up over 80% 🤔.

Googling the issue showed other people complaining of similar problems in recent months, but no solutions were provided.

I repaired both disks, deleted Time Machine local snapshots before erasing the Time Machine drive (oh so many times), but nothing seemed to work.

I should mention I am still running the last version of Catalina, 10.15.7, and that my main drive is a M2 formatted with APFS, while the backup drive is a spinner formatted in HFS+ (as APFS for Time Machine drives is only supported starting with Big Sur).

Curiously, the exaggerated size of the backup was known to Time Machine even before starting the first backup: if I went into TM Preferences’ pane before doing the initial backup and removed a folder, the estimate size stayed consistently well above my internal drive occupied space.

This morning I figured out what was causing the problem for me, and I’m posting this so that it can maybe help somebody else from Google in the future.

I have a second hard disk in my computer which I use as a clone of the main drive; obviously I keep the clone disk unmounted at all times, and certainly when running Time Machine initial backup. Nevertheless, Time Machine was producing a backup way larger than the internal drive because, while the Clone volume was unmounted, Clone - Data wasn’t, and therefore TM backed up not only the files on my main drive, but for some unexplicable reasons also all the files on Clone - Data, a different hard disk! Please be advised that Clone - Data was not visible on my Desktop, but showed as mounted in Disk Utility, and that’s the reason I didn’t notice it being mounted before (also, I did expect that ejecting Clone from the Finder also would have ejected the Data APFS volume, but clearly that’s consistently not happening for me… so both Clone and Clone - Data are now excluded from auto-mounting in fstab).

Possibly, you are thinking this makes no sense. I agree. Something must have changed in the last few months, because I never noticed this issue before last summer (so, already on Catalina) while I’ve been using this setup for years, but here I am now, with a fully-unmounted Clone hard disk and a Time Machine initial backup roughly the same size of my internal drive.