At the Guardian, John Mullan lists 10 of the best identical twins in literature.
(Hmmmm, this would not be my list - I am not a fan of the Comedy of Errors! - and both the Cheerybles and Cor and Corin seem to me more along the lines of some of the few identical twins in literature that one can think of offhand rather than being in any more substantive sense the best! A book that should be on this list and isn't: Angela Carter's Wise Children. And I have a soft spot for Barbara Trapido's twins, too. It is not surprising that Mullan hasn't read Gina Frangello or Marcy Dermansky, but he would find both of those novels well worth his time also!)