You know, one of the common characteristics of people who have been brainwashed (by a cult or otherwise) is that they absolutely do not believe that they have been brainwashed, and will fight that suggestion with all their strength, coming up with never-ending explanations for why they haven't been brainwashed at all!
OTOH, if you were brainwashed into believing another group was brainwashed, you will fight any suggestion the other group was not brainwashed and come up with an unending stream of reasons to believe they were.
Even more twisted: what if you were brainwashed into thinking that people who have been brainwashed don't believe they've been brainwashed? Would you be able to tell whether you've been brainwashed yourself or whether it's the rest of the world that's brainwashed?
The world would be a strange place... but very clean.
</tongue-in-cheek>