Full Idea
Just because one can imagine that A and B are not identical, it does not follow that A and B are not identical (think of the morning star and the evening star).
Gist of Idea
One can wrongly imagine two things being non-identical even though they are the same (morning/evening star)
Source
David J.Chalmers (The Conscious Mind [1996], 2.4.1)
Book Reference
Chalmers,David J.: 'The Conscious Mind' [OUP 1997], p.130