I agree with Katie. This can be another user with the name, the same user with a different type of account, or if you invited before and it either timed out or you removed them, then tried to invite again.
If there is a (or multiple) known emails that are associated with the user, the user can choose "forgot login" at one of the login screens, and then they should be able to input an email and get a list of all the accounts associated with that email. Maybe you will see where this name is used...if not, may be another person. (Accounts are unique)
i usually create a fname.lname_<Dept initials> to make it unique from other in the world, but still get the 1, 2, ... sometimes.