[KH II]
Ansem's back!
Except the
real Ansem is dead, and this
fake Ansem is just Riku in disguise.
... except it turns out that the
real Ansem wasn't the
real real Ansem - it was just a
fake real Ansem using the
real real Ansem's name the whole time! The fake real Ansem's name was actually Xehanort.
[KH:BBS]
... but Xehanort isn't the
real Xehanort, just a fusion of the
real Xehanort and some other bloke, who also lost his memory so fake Xehanort (who, remember, is also the fake real Ansem) doesn't even know he's the fake Xehanort!
[KH

DD]
Real Xehanort is back from the past!
And so is fake Xehanort!
And so is real Xehanort... again, but from even further back in the past!
And now everyone in Orginization XIII is also Xehanort somehow!