Hannah's right. The whole point of them going was so that the Doctor could teach his sonic the difference between a reading from a human, and one from a ganger. Then he could scan Ganger Amy to find out which she was, and block the signal from the real Amy by melting the Ganger
Did they say that in the episode? I really need to pay more attention
Not in as many words as that, but that was my understanding of the whole story arc, reading between the lines