My guess is you somehow jumbled the name when naming your computer and then setting up the owners account during install. Not knowing you'd already done this you must have created the two named Accounts in addition.
I would simply rename the active Jack account to what you want it and delete the other one. As I said earlier you cannot delete the owner's account set up first during install for security reasons, likely the Jack.Jack-Home-PC account also which carried over the PC name you gave it. So I'd rename and keep that one.