in english, before anyting is marked, senior markers sit down and read every single paper to get an idea of what sort of responses and answers to questions they are getting. then, when marked, markers sit in groups at tables of 8-10 and each person marks a bundle of 20 essays/questions from the one centre, then they are taken away and someone else marks them 1 1/2 to 2 weeks later. if 2 people plagarised the same essay, there isn't a great chance that the one person would mark the two of them - unless they were from the same centre. in that case, there is a fairly high chance that both would be marked by the same person - many schools have only 1 adv anglish class, so all their papers would be in the same bundle.
im not sure what would happen if they did though - they might produce both and tell the senior marker what's happened, but i highly doubt anything would come of it. anyway, theres basically no chance that the essay u learnt would be for the exact question they asked, so ud have 2 modify it, so they wouldnt end up being the same anyway.