Full Idea
Humans are largely made of H2O, so there could be no twin on Twin Earth, and (as Kuhn noted) nothing with a significantly different structure from H2O could be macroscopically very like water (but topaz and citrine will do).
Gist of Idea
Humans are made of H2O, so 'twins' aren't actually feasible
Source
Gabriel M.A. Segal (A Slim Book about Narrow Content [2000], 2.1)
Book Reference
Segal,Gabriel M.A.: 'A Slim Book about Narrow Content' [MIT 2000], p.24
A Reaction
A small point, but one that appeals to essentialists like me (see under Natural Theory/Laws of Nature). We can't learn much metaphysics from impossible examples.