I originally started Clash of Clans with one Game Center and my friend told me to log into his Game Center so somehow my Game Center is saved on multiple accounts so now my friend can use my account. Does anyone know how to remove my friend's Game Center while keeping my original one?
Game center is associated with an Apple ID. I do not believe that it can be shared across users. It is possible that you are still using your friends account ID. You can change your settings like so :
iOS: Go to Settings and tap Game Center. You might have to sign in with your Apple ID. OSX: Go to the Me tab and click Account > View Account.
or refer to this page.