Full Idea
If each possible world constitutes a concrete reality, then no object can be present in more than one world - objects may have 'counterparts', but cannot be identical with them.
Gist of Idea
If worlds are concrete, objects can't be present in more than one, and can only have counterparts
Source
Stephen Read (Thinking About Logic [1995], Ch.4)
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.117
A Reaction
This explains clearly why in Lewis's modal realist scheme he needs counterparts instead of rigid designation. Sounds like a slippery slope. If you say 'Humphrey might have won the election', who are you talking about?