Catalina changed the way the system is organized. First off, the format of the boot drive (and the backups in Time Machine) were changed from HFS+ to APFS. In APFS, the concept of partitions has changed. Instead of having partitions to mount as drives, there is a Container, in which Volumes can be created to share the total space of the Container. In the simplest case, there would be one container on your boot drive, with the ability to have multiple Volumes in the Container, each mounted as individual drives. However, there is a special use case for the system itself. In Catalina, what was on the default Macintosh HD drive in previous versions was divided into a set of system files and a set of User files. The System files were put in one Volume named Macintosh HD (or whatever you named the boot drive) and the user files are now in a second volume called Macintosh HD - Data, or whatever you named the boot drive with the "- data" appended. The operating system combines the two drive when it boots so that to Finder, or on the desktop, there is one icon and in that icon are all the files for both Volumes. However, Disk Utility can see that there are two volumes, as can Time Machine. So when you make a backup with TM, if you open the backups.backupdb folder and look at the backup, you will see both names. However, if you open Time Machine normally, again the OS merges the two Volumes to look like one. So if you bypass TM, Finder, etc, and look at the Container directly, there will be two volumes (actually more than two but the others are hidden), but the "normal" view is the merged, single drive view in Finder. TM sees both Volumes because it has to be able to put back to each of those two volumes the files that were there originally. So TM sees both volumes. Disk Utility sees both volumes because it is the tool used to create and remove volumes, so it has to know about all the volumes in the container.
Bottom line: Macintosh and Macintosh - Data are not duplicates, but separate volumes, both required, that are merged by the OS into Macintosh HD as it appears on the desktop and Finder.
BTW, as a benefit of the Container/Volume approach of APFS, if you want to divide a drive into multiple drives, you no longer use partitioning to create fixed size drives, you simply add a volume to the container and it will share the free space in the container with all the other volumes there. Similarly, if you want to remove a volume, you can, without impacting the other volumes in the container.
From your description, it's hard to tell exactly what you are seeing. It would help if you gave some screenshots of where you see these duplicated names. If they are what I just described, that's normal. If either Macintosh HD or Macintosh HD - Data are duplicated, that is not.
