I Have both Macintosh HD AND Macintosh HD-Data on my desktop. Macintosh HD-Data is empty. Can I delete it?

NO!

That is normal for the new APFS disk drive format that is used with Mojave, Catalina and Big Sur.

MacOS is securely installed on Macintosh HD, and your user folder and apps and data reside on Macintosh HD - Data.

However, the fact that both of those volumes appear on your desktop indicates that there may be an issue with the proper installation of the OS.

What do you see when you launch Disk Utility? In DU, click View > Show All Devices and then post a screen shot of the resulting display.

Did you recently reinstall the OS? With Big Sur, there will be a second HD - Data folder created unless you follow the correct protocol. As long as you have one showing in Disk Utility that contains your files/data, you can safely get rid of the empty duplicate folder. And, FYI, the "real" Data folder should not be showing on the desktop. But do make sure that you do have a Data folder that contains your files.

There should only be one volume, Macintosh HD, on the desktop. One is extra.

Please go to Disk Utility, and select "View" > "Show All Devices". Select each device (or entry shown), and take a screenshot of it and upload them all here. 

Jack

Select the very last Macintosh HD - Data (the one without a home folder icon), and select delete (“-“) in the toolbar.

Jack

You’re welcome.