To answer the question about the existence of multiple identities, it would appear useful to define and meaning of "individual." It would supposedly differ.
Individual as defined as:
--different or separate: separate how? from what?
--intended for the use of one person only: what is a personhood?
--characteristic of a particular person: unique combination of which traits?
--distinguished or distinctive style: extension of which qualities?
--conscious or unconscious beliefs
The biologial, bichemical sides (DNA) could be easier to pin down in an individual than the emotional, psychological, metaphysical and other facets of identities which tend to be grouped (i.e., Meyer's Briggs Personality Test and DSM-IV Psychiatric Manual).
Its worth mentioning some individuals believe themselves to have multiple identities, and these people aren't all MPD psychiatric patients. That is, these people may perform different social roles, exhibit different talents, consider themselves part of social, cultural, economic, political, hierarchical or other associations or organisations. Some are public and others are private. Some are conscious associations and others are unconscious.