Multiple Macintosh HD after reinstall

Re do the erase and reinstall but erase your disk properly.

Boot to your Recovery HD or bootable USB whichever method you use for reinstalling.

Open Disk Utility, click on View in the menubar and select Show All Devices.

Highlight the Disk (Apple SSD or similar) not any of the indented Volumes.

Click Erase.

Give the Disk a name.

Format: APFS.

Scheme: GUID Partition Map.

Click Erase.

When Done quit Disk Utility.

Reinstall the OS.

I believe what happened is when you decided to erase and reinstall Big Sur you ONLY selected the Macintosh HD and erased it - leaving the Data volume. Then when you reinstalled macOS it created a 2nd Data volume. You can get rid of the 2nd Data volume via Disk Utility but you need to be careful and select the correct one. You will need to look into each volume to see which one has your data.

Always erase the VOLUME GROUP when you want to erase your Macintosh HD. This will erase both Macintosh HD and Macintosh HD - Data volumes. In Disk Utility you should see a button to Erase Volume Group when in Recovery Mode.

you need to keep the one that is at mount point /System/Volumes/Data. the one mounted /Volumes/Data can be removed.