Full Idea
Possible worlds are just abstract objects that play a certain role in philosophers' talk about modality. They are ways things could be. That's why there are no two abstract possible worlds which are qualitatively identical. They count as one world.
Gist of Idea
Possible worlds must be abstract, because two qualitatively identical worlds are just one world
Source
Ned Markosian (A Defense of Presentism [2004], 3.10)
Book Reference
'Persistence: contemporary readings', ed/tr. Haslanger,S/|Kurtz,RM [MIT 2006], p.329
A Reaction
Brilliant! This looks like the best distinction between concrete and abstract. If two concreta are identical they remain two; if two abstracta are identical they are one (like numbers, or logical connectives with the same truth table).
Related Ideas
Idea 10516 A realistic view of reference is possible for concrete objects, but not for abstract objects [Dummett, by Hale]
Idea 10519 The abstract/concrete distinction is in the relations in the identity-criteria of object-names [Hale]