The reply I have seen to this argument is that the gospels have just enough differences to be considered not copied, but enough similarities to be considered reliable.
Let me put it another way. If I gave a lecture to my class and asked them to write an essay about the lesson I will get as many stories as I have students. If the stories were all exactly the same I would rightfully say they cheated. But if they were listening I would expect the 25 or more different stories to be basically similar.