Why does OS store all files twice? (using twice the storage on HDD)

You are mistakenly assuming that files are stored twice. They aren't.

The software you are using (with which I'm not familiar) is probably giving you different views into the contents of your drive, and you are assuming they are duplicated.

A default configuration of macOS doesn’t store all files twice. macOS does have more than one Applications folder and Library folder, but the content would be complementary, not duplicate.

I don’t have an opinion on the tool that you used, but you may ask the developer about features and results that are unclear.

Also note that '>system/volume/data/users' refers to a disk volume named ‘Data’, which is probably a different disk volume than the boot volume. If that is a separate partition, then that is how you configured that computer, with duplicates and all.

Kandinski wrote:

When I use a software that analyses space used on my Hard Drive (like Space Gremlin), it finds all files twice. Once under: '>system/volume/data/users' directory and then again under: '>users'... and '>Applications'.

So space used in total is: 808gb

+ 303gb = System (including Users: 229gb + applications: 39gb + library 7gb + other)

then separately:

+ 275gb = Users: 229gb + >Applications: 39gb + >Library: 7gb

then

+ remainder = restricted space

You are looking at the same data. Try different software if in doubt, like OmniDiskSweeper.

From the Finder you can highlite the Macintosh HD and command i will open the info pane—here you can see available and used storage totals.

About the read-only system volume in macOS Catalina

https://support.apple.com/en-us/HT210650