Macintosh HD - Data seems to duplicate data?

The data are not duplicated. The System volume contains a read-only installation of macOS. The Data volume is invisible and contains all your data. Special aliases, called "firmlinks," point from the System volume to the Data volume. Thus, both volumes appear the same but they aren't. This is quite normal.

Now, if you have two Data volumes, then there is a duplication but not done by macOS. It was done if you did not erase the drive correctly before reinstalling macOS resulting in two Data volumes. If you open Disk Utility and set the View dropdown menu to "Display all Devices" then you will be able to see if you have two Data volumes.

That's because they are both the same thing. You are double-counting. Yes, you can ignore that. However, you cannot ignore have low disk space. That can become a huge problem.

What is This Other Category in Storage Display

What is 'Other' and 'Purgeable' in About This Mac?

"Other" contains any of the following types of files. They cannot necessarily be deleted wholesale. Furthermore, “Other” is not an actual, physical file. Rather it’s an arbitrary category to which the files are assigned for display convenience.

• System temporary files
• macOS system folders
• Archives and disk images (.zip, .iso, etc.)
• Personal user data
• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)
• Cache files
• Fonts, plugins, extensions
• Hidden files
• Other files that are not recognized by a Spotlight search

Get Correct Storage Information

Do not use the information from the Storage section of the About This Mac dialog. Ignore the Storage information as it is typically wrong. To find out the correct information for any disk: Select a Desktop disk icon. Press Command-I to open the Get Info window and look at the topmost panel displayed. You will find the disk information displayed for Capacity, Available, and Used. If you have more than one disk/partition then repeat for each one on your Desktop.

The categories found in the Storage section of About This Mac is simply an arbitrary way of displaying files on your drive. There are no such categories actually on the drive.

How to Free Up Space on The Hard Drive

1. You can remove data from your Home folder except for the /Home/Library/ folder.
2. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.
3. Also, see Freeing space on your Mac OS X startup disk.
4. What is 'Other' and 'Purgeable' in About This Mac?
5. Files That Make Up the 'Other' Storage Category, and How to Remove Them
6. Free up storage space on your Mac.
7. See Where did my Disk Space go?.
8. Be sure to Empty the Trash to recover the space.
9. Replace the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.
10. Use OmniDiskSweeper or GrandPerspective to search your drive for large files and where they are located.