Question: Q: Using Terminal to copy home folder and the destination copy is much larger

Is data being duplicated somehow?

Maybe. If you have an iPhotos and a Photos library, there are hardlinks (multiple directory entries that point to the same storage). There can be other hardlinks situations on macOS.

The 'cp' command does not know, and does not care if you have hardlinks. It sees a directory entry and copies it. You get a duplicated file.

macOS has the ability to compress stored files, but the 'cp' command is not going to trigger that compression, so a file may expand to use more space.

Depending on which version of macOS, there may be 'sparse' files, where large ranges of zeros in the file, do not even have allocated storage. So there may be files with islands of data with nothing allocated in-between. the 'cp' command does not know this, and allocates storage for those holes. NOTE: This is less likely to happen, as most consumer files do not take advantage of sparse files.

Basically the 'cp' command is rather simplistic with regards to how files are actually stored in a file system. Some of that is because the standard Unix file system API's does not make that information available to utilities such as 'cp'.

You might look into the 'ditto' command. It says it can preserve hardlinks. It also says it can provide HFS compression.

And, no I do not know what command you should use. I know ditto exists, and I can read the man page to see it says it support hardlinks and hfs compression. That does not mean I am skilled at using 'ditto'. Google would be your friend in this case.