How to derive Hash functions from other functions?

How to derive Hash functions from other functions?

For instance, suppose that H : {0, 1}∗ → {0, 1}n is a collision resistant hash function. Define a new hash function by G : {0, 1}∗ → {0,1}n such that G(x) = H(H(x)). Prove that G is also collision resistant. 

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