I loved reading Clan of the Cave Bear books.

I have considered the DNA memory angle. I think it has validity. I also think the Jungian collective unconscious of archetypes has validity. I believe in straight-forward reincarnation, as well. Perhaps these concepts all fit together similar to the way the physical world does with spirit.

Meanwhile, I had a couple of ideas as to why many regressed people believe they are the reincarnation of someone famous.

One possibility is that a soul is like a pie that can be slivered up and exist in numerous containers. So a particularly prominent/strong soul, such as the one that produced Cleopatra for example, could be reincarnated in many bodies--whenever, including at the same time.

Another theory is along the lines of the collective unconscious, where we tap into a huge pool or filing system of all thoughts and all lives. From here we could experience anyone's previous (or current, or future) life. I also think this accessible "whole" is where psychic information comes from.

Another thought about reincarnation of popular historical figures is that because the lives of these people are dwelt upon by so many others, their path is highlighted, strengthened, easy to tap into, and may also be archetypical of certain eras.