Answer:
I have not so far encountered cases where persons have developed multiple personalities solely through the practice of astral projection, that have manifested in their waking lives as personality disorders. However, during OBE's and dreams, different aspects of a projector's personality may indeed take prominence, and often will.
The most common personality alterations during OBE come from the uninhibiting effect that projecting into the astral dimension has upon people. This causes projectors to manifest suppressed desires and other hidden aspects of their subconscious. Exaggerated lust and lack of conscience are probably the most common astral problems in this respect.
As for fixing this, if it became a problem, I would suggest the help of a good psychotherapist.