Anakin Skywalker and Darth Vader are two different persons inhabiting the same body, who agrees with such a view. The script and novelisations also show that such duality appear to exist between Palpatine and Sidious. In Disney canon, Kylo Ren also appears to consider himself different from his birth identity.
This suggests that, from a philosophical standpoint, one's dark self is different and separate from one's natural birth self. Is this specific only to these three, or is it true of all Sith (Kylo's could be just yet another instance of Vader worship), all dark siders or even all living things in general?
Edit: I included Kylo Ren because it's another case of such a duality that we can observe going on in the novelisations. However, I am interested to know of examples and counterexamples in past and future Sith, including ones in Legends, where such dual identities exist.