Duplicates on Catalina MacHD & MacHD-Data volumes

You should have an OS volume and one Data volume. If you have more than one Data volume, it is leftover from a reinstall.

Look in Disk Utility for your Volumes.

The orphan Data Volume will have a mount Point at /Volumes.

The current one will be mounted to /System/Volumes/Data

Files inside of the Container are not duplicates, even if they look like it, unless you have more than one Data volume

Actually, those files ARE in the MacHD - Data. It's the nature of Finder that it will never display the "- Data" portion of the APFS volume names that are accessible to the user.

It's a Finder thing.

But if you use Disk Utility, Terminal, or any other drive utility you can see the long names of the various volumes that your Mac is using.

ALL of your data is stored in the "Macintosh HD - Data" volume, but the Finder hides this details from users, so you only see one volume.

There is no duplication, because the "Macintosh HD" only contains system files.

Open Disk Utility. If you see something like this:

then everything is ok.

If you see TWO "Data" volumes, then as Barney said you can and should delete one of them, as instructed.

The files inside which container? The MacHD or the MacDisc- Data?

Neither are a Container. Those are two Volumes that reside inside the same Container.

There are numerous files and folders in the two User folders that are identical. They are the same size and both open as a separate but identical documents,

Do you see a drive named - Data in the Finder? If so, you did not reinstall correctly and you have an orphan Data drive from the previous installation. You can remove that orphan in Disk Utility as I noted above.

When a Mac's drive if formatted in APFS, what we used to call "partitions" are now called "containers." In turn, containers can contain volumes. With the very latest versions of macOS, the OS itself is resident in a read-only "Macintosh HD" volume. All of your apps & user files are in a separate volume, called: "Macintosh HD - Data". These two volumes are considered as a volume group. As mentioned, there should only be one volume group with two volumes inside a single container for macOS. Also, as mentioned, the Finder app (in an attempt to prevent confusion) "labels" the data volume just "Macintosh HD."

So, if you are using the Disk Utility, if you see multiple "- Data" or other named volumes, these are not a part of the macOS volume group, but got there by "accident" from something like a failed OS install or update.

Not sure if you are using "MacHD" as a short-hand for "Macintosh HD" (which is the default name) ... but if it actually is "MacHD," it would have been renamed somehow. We also don't know how you got to this situation in the first place. The fact that you stated you also had a volume named UserBU indicates some type of user interaction as well. We could try to "piecemeal" your Mac's drive back together again, but at this point, I think you will be better served by first completely erasing the drive, and then, install a fresh copy of macOS ... at least, that's what I would do.