Isn’t this phrase redundant?

The passage below is from A History of the Index by Dennis Duncan.

In these lines, Gertrude may be thinking not just of how readers use indexes, but also how books were physically arranged. In many, though by no means all, fifteenth- and sixteenth-century printed books, the index was bound at the front of the work (as is still the case with tables of contents). It will, in time, undertake a slow migration to the position it occupies today at the back of the book, a move that is accomplished by the early eighteenth century, so that the Grub Street Journal, in 1735, can muse that ‘an Index … in most of the books I ever saw printed before 1600, stood in the same place where our Preface does now’. But this anonymous reflection overstates the case. Indeed, binders or their customers could decide for themselves at which end of a book their indexes should go, since they were generally printed on separate gatherings from the rest of the work, often with signatures that stand outside the usual alphabetical order, using other signs from the printer’s case such as asterisks (*1, *2, *3, etc.). The fact remains, however, that in the days of the early printed book, the back-of-book index was often the front-of-book index. But the idea of the index coming first is more important as a figurative statement than a bibliographical fact, and this is what comes across loud and clear in the Shakespeare examples: the index is the baby figure of things to come.10

10 Christopher Marlowe – one of Shakespeare’s great influences and a possible early collaborator – also uses the image of the index in this sense in his long poem Hero and Leander: ‘Therefore even as an index to a book, / So to his mind was young Leander’s look’ (II.129–0). In other words, Leander’s thoughts were written all over his face, but the precedence – the face is in front – is already there.

In Middle English index have a meaning as preface. So...

Marlowe’s epic poem says (in my rough paraphrase) ‘Therefore just as a preface is to a book,/ So is young Leander’s face to his mind’. (Am I right?)

Then, comes this passage, ‘In other words, Leander’s thoughts were written all over his face’ like a preview of preface(index).

I was OK with the book till this passage.

But the underlined passage ‘but the precedence – the face is in front – is already there’ seems awkward to me. ‘Face’ and ‘precedence’ is the same information referred to just before them, I mean ‘Leander’s thoughts were written all over his face’ seems just the same as ‘the precedence – the face is in front – is already there’.

Then why is same information added one more time?

That is not all.

Why two same informations are connected with ‘BUT’ that connects two opposite ideas?

Thanks in advance.