Books V, VI, and VII of the Nicomachean Ethics are identical to Books IV, V, and VI of the Eudemian Ethics.
Why did the author(s) write two pieces so similar.
We can't tell from 'the common books' whether these were transposed from EE to NE or the other way round. My impression is that NE replaces EE. On a range of matters Aristotle did not change his mind; and these matters are expounded in the common books. I am persuaded that both EE and NE are authentically Aristotelian works in whatever state they were left. 'Books' is simply a term of convenience.
The 'uncommon books', those that differ between EE and NE, show differences which to mind suggest that NE is the more sophisticated and later work. EE and NE are similar because they represent, if I am right, different stages of development in Aristotle's thinking about ethics but without any drastic or decisive rupture. But they are only similar and not identical because though there is much continuity but there are also divergences. I mention two points.
▻ EE, NE & THE COMPLEXITIES OF THE MORAL LIFE - IMPLICATIONS FOR THEORY
In NE II.2 1104a1-10 Aristotle observes :
But this must be agreed upon beforehand, that the whole account of matters of conduct must be given in outline and not precisely, as we said at the very beginning that the accounts we demand must be in accordance with the subject-matter; matters concerned with conduct and questions about what is good for us have no fixity, any more than matters of health. The general account being of this nature, the account of particular cases is yet more lacking in exactness; for they do not fall under any art or set of precepts, but the agents themselves must in each case consider what is appropriate to the occasion, as happpens also in the art of medicne or of navigation. (J. Barnes, ed., The Complete Works of Aristotle, II, Princeton : Princeton University Press, 1984, 1743-4.)
This sense of the inadequacy of general theory and the inherent inexactness of ethics is not expressed in EE, where ethics is treated as a matter that can be encompassed by a general theory This theory can be learnt first and particulars - specifically appropriate actions and states of emotion - readily recognised and subsumed under it. This is top-down : from the general to the particular.
In contrast NE is bottom-up : some rules of thumb can be formulated but they are no more than that. A formula for the 'mean' (to meson, mesotes) can be set out : courage is the mean between cowardice and rashness, friendly civility a mean between surliness and obsequiousness and so on. But only experience can enable you to recognise what courage is as distinct from cowardice and rashness : and what courage requires will vary from situation to situation. All this is learnt, not from theory but 'on the ground' in the actual, shifting and uncertain circumstances of life and by observing the behaviour of the phronimos, the practically wise person who has followed this path already. EE offers theory where NE is alert to the real complexities of the moral life to which theory is inadequate. To my mind this suggests that Aristotle had learnt more about the moral life after writing EE and that this extra knowledge, this finesssed perspective on morality, is embodied in NE.
▻ EE, NE & APORIAI
Another point is that EE leaves aporiai - puzzles or lacunae - to which NE supplies answers. Here is one example :
the brief account of the acquisition of virtue that we receive in the EE leaves itself open to the following puzzle. On the one hand, we are told that the best of the soul's activities result from the exercise of moral virtue. On the other hand, we are told that it is by performing these very activities that moral virtue is acquired. But if the activities that are necessary to acquire virtue are the very activities that are produced by the exercise of the virtues, then it becomes unclear how one can acquire virtue if one is not already virtuous. That is, how can one engage in the right kind of activities if these activities are themselves the result of exercising moral virtue?
Aristotle does not address this issue in the EE, but in NE book 2, chapter 4 he outlines the dilemma and offers a response in which he clarifies the relationship between actions and emotions that are virtuous and virtuous dispositions. Although actions are just when they are the sort of actions that just and temperate people would perform, a moral agent is not just or temperate simply because he performs such actions. Rather, the agent is just or temperate when he consistently performs such acts in the way in which just and temperate people do, that is, when (1) he acts knowingly, (2) he deliberately chooses the act for its own sake, and (3) the act springs from a fixed and permanent state of character.35 An integral part of acquiring a virtuous character, therefore, is developing the proper affective orienta tion to virtuous actions.36 But in the first four chapters of NE book 2, Aristotle is clear that we develop these affective responses by repeat edly performing the right kinds of actions. It is only by acting in dangerous situations and developing the proper reactions of fear or confidence, for instance, that we become courageous or cowardly. The first step to becoming just and temperate, therefore, is to perform just and temperate actions. ( Alex John London, Moral Knowledge and the Acquisition of Virtue in Aristotle's "Nicomachean" and "Eudemian Ethics"', The Review of Metaphysics, Vol. 54, No. 3 (Mar., 2001), pp. 553-583 : 565-6.)
Probably the exact relation of EE to NE will never be established beyond reasonable controversy. But reasonable argument is possible and this is what I hope I have offered.
My own theory is that Nico (son of Aristotle) asked Aristotle to write him a book explaining Ethics.
Aristotle didn't start from stratch, but gathered a lot of his already written out works (whom he penned himself) and told his "scribes" to copy them in one go, neatly, word for word so Aristotle could give it to Nico. Later when Eudemus asked for a book explaining Ethics. Aristotle did the same (thinking Eudemus might enjoy a diffrent order) so he told his scribes to re-write Book IV as Book V for Eudemus.
So my thought is that Aristotle used his scribes as copy-paste. But wrote both books for his students.