Prove that function G is not a pseudo-random generator
You can construct a discriminator which applies G' on the first half of the output in question and compares it to the second half. If they match claim it came from G'' otherwise call it random. 

This discriminator has no false negatives and a false positive rate of $2^{-n-2}$. 

The ability to build an efficient discriminator with non negligible success probability proves G'' is not pseudo random.