The Bobbsey Twins, of course! By Laura Lee Hope. Just beware, if you are buying the older books instead of re-issues, they had some questionable content. I recall one visit to the Southern states that had some terrible stereotypes.
I Love You the Purplest
by Barbara Joosse is a beautifully illustrated picture book about a mom and her two sons and the lovely way she recognizes what is special about both of them. I don't think it says that they are twins, but they could be...
The Man in the Iron Mask by Alexandre Dumas
Are stories about look-alikes okay?
The Prince and the Pauper by Charles Dickens
Follow Mothering