Two Macintosh HD volumes of identical size listed on my internal HD???

No, not identical, sicw many OS versions ago Applev has split the Boot volume in two, one Read Only volume called Macintosh HD, & a second Users volume called Macintosh HD - Data, & the advantage of the new APFDS is that all volumes can share the Free Space as needed.:)

TrexoRT wrote:

https://discussions.apple.com/content/attachment/e0834ded-37a9-4865-80b1-6ad942e9f7e3

My iMac System Report shows the following: Backup of iMac (9) 84.98 GB (this is to an external disk); Macintosh HD 144.74 GB; Macintosh HD - Data 144.74 GB; and Photo Edit Files 3.94 TB (external disk). Why are there 2 Macintosh HD volumes of identical size listed? Can someone please explain?

[Edited by Moderator]

You can get another concise look at the Drive structure from the Terminal.app copy and paste:

diskutil list internal

Your user data is the "Macintosh HD- Data" volume

below is the big picture schematic:

TrexoRT wrote:

diskutil list internal
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk1         1.0 TB     disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +1.0 TB     disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD            11.2 GB    disk1s1
   2:              APFS Snapshot com.apple.os.update-... 11.2 GB    disk1s1s1
   3:                APFS Volume Macintosh HD - Data     841.9 GB   disk1s2
   4:                APFS Volume Preboot                 2.5 GB     disk1s3
   5:                APFS Volume Recovery                1.3 GB     disk1s4
   6:                APFS Volume VM                      20.5 KB    disk1s5

re: reply to leroydouglas: it is an Intel Mac, 27 inch

Ok Intel I can post the Intel Schematic it will show the same new structure BDAqua described.

Your terminal output looks normal for Intel.

Are you having some issue?

TrexoRT wrote:

Do you know if it’s using twice the storage space? I’m running out of disk space!

No the APFS file format has nothing to do with Free Space or the lack of it.

To free up some Storage Space, carefully read and study > Free up storage space on Mac - Apple Support

TrexoRT wrote:

Do you know if it’s using twice the storage space? I’m running out of disk space!

No it is not—

Not to mention you can not change the drive structure.

Where you have r/w permission is your User data ie your Home folder—to add or remove.

👍🏽

No it.s not using twice the space, it’s the free space that is identical for both, not the used space.

your Terminal output shows the used… 11.2 & 841.9 GB.

look for local Time Machine backups.