I deleted some user and group accounts-need to reuse the names

I deleted a group account and a couple of user accounts that I now need to use the names of again. However, when I go into System Preferences\Users & Groups and attempt to created the needed accounts, I get the message that an account already exists with those names. As far as I can tell, those accounts are completely gone. They are not in the Users & Groups area and are not in the Users directory at all. They were deleted quite some time ago (months, if not longer). How do I get rid of whatever thinks they are still there so I can reuse the account names again? I need to use specific account names for the software I am trying to use.