How to work simultaneously on a few branches

This is a follow-up on this question on creating branches.

It strikes me as odd that I would still work on one repository because the files on my local machine will be a weird mix of different experiments.

I would imagine the best-practice method is to duplicate the repository and work in different folders on my computer for each branch -- but I don't know how to set this up. I have my current repository at Documents/San/CompProj so what are the commands I'd use to create a new repository tied to a different branch on a different local folder?

Git is fairly new to me so I'd love any corrections you can make on what I'm assuming/asking above.