Dec 02 2008
Generally, character overlap is problematic. If two characters are interchangeable or perform the same role in your story, removing one is probably wise. Having a smaller cast-size tends to save space, improve characterization and facilitates tighter scenes.
Twins (and triplets and quadruplets, etc.) tend to be either indistinguishable copies or slightly modified versions of the same mold. If they’re indistinguishable, then the second twin is heavily redundant with the first and either can be easily removed.
On the other hand, some twins have only a slight difference, usually along a single character trait. For example, one is optimistic and the other is downcast, or quiet vs. loud/outgoing, etc. There are a few problems with that. First, one-dimensional differentiation is typically flimsy and shallow. Why not just make them distinct characters? Second, it’s generally harder for readers to keep twins apart, particularly identical ones.