Yosemite Updates From App Store Not Working

I recently bought a new iMac and installed Yosemite on it, but for some reason whenever I try to update I just get this message:

"[Name of App] was purchased using the App Store on another computer. To use [Name of App] on this computer, sign in using the Apple ID and password used to purchase it."

Nope. My Apple ID remains the same as it was on my old iMac, and I don't remember ever having another one. If anybody knows how to sort this out, I'd be very grateful as I haven't been able to update anything in about three months.