Hi! I reinstalled macOS Catalina and now I have two Macintosh HD - Data and Macintosh HD - Data - Data. It also eats 30 gb of space on my 120 Gb SSD. What to do? How ti delete one one of them and free the space?

Unless you erase the entire drive, an install will simply keep creating another Mac HD - Data drive. I guess at this point, it would not hurt to erase the Data volume (as long as you have a backup) and then see if you can choose the entry "SSD Media" (top most entry) to erase.

A new feature of MacOS Catalina is that it intentionally creates one "Macintosh HD" Volume that contains only the Read-only portion of MacOS.

All Read/Write data including the /Users directory that contains user data under your login-name are placed in the new Volume, "Macintosh HD-Data".

It is important to understand that multiple Volumes sharing the same physical disk are now a normal feature of Apple File System (APFS).

For some unknown reason, your installation attempts have resulted in two of these Read/Write Volumes.

If you can determine which is "the right one" you can delete the other.

If you can not figure this out, you may need to use your Trusted Backup. To proceed, you need the Backup created right before you started the Install process.

I would not suggest deleting anything individually at this point. There is definitely a borked/duplicated install.

Catalina does have two volumes: Mac HD and Mac HD - Data, the first is the system and read only; the second has full access read/write.

So, I would suggest using recovery and its Disk Utility to erase the drive completely and then reinstall from scratch. Make sure you have a backup of all of your files first.

Glad you got it figured out!