This technique for creating an administrator account in Vista is described here but your symptom of having two adminstrator accounts seem to contradict a claim made in this article.How to activate the hidden Vista Administrator Account
"One puzzle remained, if, when you initially logon to Vista, can you create (as opposed to activate) an account called Administrator.
John Wolfe came up with the answer. 'When I tried to name the original account as Administrator. I was told that the "Account already exists" '.
My point is that you cannot have two accounts with the same name, and unlike previous operating systems, before you can logon to Vista you must first create an account."