Full Idea
An extremely vivid person might have no counterparts, and Da Vinci seems to me to have more than one essence. Bertrand Russell is clearly the counterpart of at least three distinct persons in some more plausible world.
Gist of Idea
Unusual people may have no counterparts, or several
Source
David Kaplan (Transworld Heir Lines [1967], p.100)
Book Reference
'The Possible and the Actual', ed/tr. Loux,Michael J. [Cornell 1979], p.100
A Reaction
Lewis prefers the notion that there is at most one counterpart, the 'closest' entity is some world. I think he also claims there is at least one counterpart. I like Kaplan's relaxed attitude to these things, which has more explanatory power.