MacOS: Two Calibre Volumes

[elektronaut]

Hi, just noticed that on MacOS

$ diskutil list

shows me two Calibre Volumes.
Why do they exist, and if they should not be there, how can I get rid of them?

Here's the output:

/dev/disk3 (disk image):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        +436.2 MB   disk3
   1:                 Apple_APFS Container disk4         436.1 MB   disk3s1

/dev/disk4 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +436.1 MB   disk4
                                 Physical Store disk3s1
   1:                APFS Volume calibre-5.9.0           399.4 MB   disk4s1

/dev/disk6 (disk image):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        +436.5 MB   disk6
   1:                 Apple_APFS Container disk7         436.4 MB   disk6s1

/dev/disk7 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +436.4 MB   disk7
                                 Physical Store disk6s1
   1:                APFS Volume calibre-5.10.1          399.7 MB   disk7s1

Any help is greatly appreciated!