I wasn't able to edit a username (the email had been entered incorrectly) so I deleted the user and tried to add the user again, this time with the correct email address. However, I keep getting the "Duplicate User" error. How can I fix this?
Try logging in with that account, it is likely that a separate account was created when the user tried to log in originally. It didn't log into the correct company since the username was incorrect, and instead created a brand new Company just for that user.
Log in as that user, go to the Settings page, and delete the Company. Once you have successfully deleted the Company (which also deletes the user), try adding the user to the correct Company again, and it should work as expected.