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ex1) John is as strong as Bill is strong (X)

ex2) John is as strong as Bill (is) (OK)


Why is ex1) ungrammatical? It looks grammatically correct but it isn't

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(1) is not strictly speaking ungrammatical. We can say e.g. "John is as patient as Bill is stubborn", so you can see that the grammatical pattern works. The issue with (1) is the unnecessary repetition of an adjective that we expect to be omitted.