Multiple Personality

It is possible for a human brain to hold multiple personalities, it is a
well understood neuropsychological condition connected with some of the
natural higher cognitive functions of the brain, connected with early
development, socialisation and learning processes. Multiple personalities
are easy to detect using a simple brain scanner.

Where the condition becomes pathological there are a number of effective
treatments that will re-balance the mind's function and allow a fully
integrated personality.

Mind-wipe also removes multiple personalities.

There are some people who prefer to have multiple personalities, and in
some cases actually choose to have the condition induced. There is no
evidence that this creates long term harm, though it makes such
individuals tricky to socialise with.