Duplicate files at / and System/Volumes/Data/

A few weeks ago I updated an iMac with SSD to Catalina. After a while I noticed that the system drive had less than 5% space left. Not a healthy situation for the SSD.

A quick review it appeared that all folders from the root / were duplicated in at /System/Volumes/Data/

(base) iMac:.ssh user$ ls -ali /
total 33
                  2 drwxr-xr-x   24 root  admin   768  7 apr 13:31 .
                  2 drwxr-xr-x   24 root  admin   768  7 apr 13:31 ..
1152921500311902268 -rw-rw-r--    1 root  admin  8196  7 apr 14:29 .DS_Store
1152921500312397577 d-wx--x--t    3 root  wheel    96  7 apr 13:31 .Trashes
1152921500312397156 lrwxr-xr-x    1 root  admin    36 19 mrt 09:16 .VolumeIcon.icns -> System/Volumes/Data/.VolumeIcon.icns
1152921500311902444 ----------    1 root  admin     0 13 dec 23:03 .file
1152921500311879696 drwx------    9 root  admin   288  7 apr 16:26 .fseventsd
1152921500312397157 drwxr-xr-x    2 root  wheel    64 13 dec 23:02 .vol
         8633670793 drwxrwxr-x+ 239 root  admin  7648 11 apr 16:07 Applications
         8633649746 drwxr-xr-x   82 root  wheel  2624 20 mrt 01:00 Library
1152921500312397546 drwxr-xr-x    3 root  wheel    96 19 nov  2017 Preboot
1152921500311902446 drwxr-xr-x@   8 root  wheel   256 23 jan 13:49 System
            1704596 drwxr-xr-x    5 root  admin   160 23 jan 13:47 Users
         8633673123 drwxr-xr-x    7 root  wheel   224 14 apr 21:12 Volumes
1152921500311902269 drwxr-xr-x@  38 root  wheel  1216 23 jan 13:58 bin
         8633673124 drwxr-xr-x    2 root  wheel    64 13 dec 23:07 cores
                307 dr-xr-xr-x    3 root  wheel  4509 14 apr 20:20 dev
1152921500311902445 lrwxr-xr-x@   1 root  admin    11 19 mrt 09:10 etc -> private/etc
1152921504606781440 lrwxr-xr-x    1 root  wheel    25 14 apr 20:20 home -> /System/Volumes/Data/home
         8633673122 drwxr-xr-x    2 root  wheel    64 13 dec 23:10 opt
         8633649229 drwxr-xr-x    6 root  wheel   192 19 mrt 09:18 private
1152921500311902342 drwxr-xr-x@  63 root  wheel  2016 19 mrt 09:16 sbin
1152921500312397161 lrwxr-xr-x@   1 root  admin    11 19 mrt 09:16 tmp -> private/tmp
1152921500311879711 drwxr-xr-x@  11 root  wheel   352 19 mrt 09:16 usr
1152921500312397153 lrwxr-xr-x@   1 root  admin    11 19 mrt 09:16 var -> private/var

(base) iMac:.ssh user$ ls -ali /System/Volumes/Data/
total 3458
1152921500311879682 drwxr-xr-x   31 root       wheel      992 19 mrt 09:28 .
1152921500312303809 drwxr-xr-x    3 root       wheel       96 13 dec 23:02 ..
         8611277339 -rw-rw-r--    1 root       admin    10244 17 mrt 20:25 .DS_Store
                 25 -rw-rw-r--@   1 root       wheel    15364 30 aug  2010 .DS_Store (of old Mac)
                 26 d--x--x--x    9 root       wheel      288 14 apr 20:20 .DocumentRevisions-V100
                 19 dr-xr-xr-t@   2 root       wheel       64 26 mrt  2017 .HFS+ Private Directory Data?
               4183 drwx------    2 user       staff       64 11 dec  2016 .IABootFiles
         8618393312 -rw-r--r--    1 root       wheel      992 16 sep  2019 .OSInstallerMessages
               4185 drwxr-xr-x    2 root       admin       64 21 okt  2016 .PKInstallSandboxManager
               4186 drwx------    2 root       admin       64 19 mrt 00:07 .PKInstallSandboxManager-SystemSoftware
            3473871 drwx------    5 root       wheel      160 26 mrt  2017 .Spotlight-V100
         8633649745 -rw-r--r--    1 root       wheel  1729962 19 mrt 09:17 .TempReceipt.bom
            3479439 d-wx--x--t    3 root       wheel       96 26 mrt  2017 .TemporaryItems
            3693106 d-wx--x--t    3 root       wheel       96  7 apr 13:31 .Trashes
         8611277340 ----------    1 root       admin        0 26 feb  2019 .file
            3472750 drwx------  219 root       wheel     7008 14 apr 21:14 .fseventsd
         8633649744 -rw-r--r--    1 root       wheel      424 15 jan 05:41 .installer-compatibility
         8611277341 drwxr-xr-x    2 root       wheel       64 15 apr  2019 .vol
         8633670793 drwxrwxr-x+ 239 root       admin     7648 11 apr 16:07 Applications
            1140515 lrwxr-xr-x    1 root       wheel       42 11 dec  2016 Developer -> /Applications/Xcode.app/Contents/Developer
         8633649746 drwxr-xr-x   82 root       wheel     2624 20 mrt 01:00 Library
         8633667995 drwxr-xr-x@   3 root       wheel       96 14 dec 02:25 System
            1704596 drwxr-xr-x    5 root       admin      160 23 jan 13:47 Users
         8633673123 drwxr-xr-x    7 root       wheel      224 14 apr 21:12 Volumes
         8633673124 drwxr-xr-x    2 root       wheel       64 13 dec 23:07 cores
                  3 dr-xr-xr-x    2 root       wheel        1 14 apr 20:26 home
         8633670084 drwxr-xr-x    2 root       wheel       64 13 dec 23:03 mnt
         8633673122 drwxr-xr-x    2 root       wheel       64 13 dec 23:10 opt
         8633649229 drwxr-xr-x    6 root       wheel      192 19 mrt 09:18 private
         8633649412 drwxr-xr-x    2 root       wheel       64 13 dec 23:03 sw
         8633649414 drwxr-xr-x    5 root       wheel      160 23 jan 13:35 usr

The filesystems appear to be linked. When I remove a file from /Users/user/Desktop/ it is also removed from /System/Volumes/Data/Users/user/Desktop/

Is it default Catalina firmlink behaviour or is there a way to unlink the two filesystems and reclaim most of the diskspace by deleting one or the other?