Is it possible to duplicate a Time Machine back up HD?

If the TM backup is to a drive formatted in APFS, then only (& only potentially) can you use a bit-by-bit clone of the drive that may work. Otherwise, you cannot simply just create a copy.

AlWeir's suggestion to obtain a second drive would be the best solution for redundancy. You can then then configure TM to make subsequent backups to each of these drives alternately.

It is not possible to copy or duplicate a Time Machine back up. Get a second drive and Time Machine will back up to each alternating between the drive.

None of the popular cloning software will duplicate an APFS formatted Time Machine drive to another drive. Tested that with CCC and another product with no success whatsoever.

In would buy another drive if you can afford it and create your redundancy.

Just to provide you with a bit more background on TM backups and APFS formatted drives, please take a look at the following article: What can you do with Time Machine backups on APFS? - The Eclectic Light Company

Yes, thank you. I'm very familiar with CCC, and I concur that it, amongst a number of other commercially available cloning solutions will not work. It's also why I mentioned that it would "only potentially" work to the OP. My guess is that only a professional data service would be able to do this ... and, most likely, only at an astronomical price to not be worth it.

You're welcome!