Little by little, English has accumulated some odd phrases. This group consists of a noun followed by a preposition, then either the same noun repeated, or a fixed, related noun.

Some examples:

N by N indicates succession, as in house by house, little by little, and word by word.
N for N indicates matching or exchange, as in pound for pound, dollar for dollar.
N after N indicates succession, as in day after day.
N on N or N upon N indicates succession too, as in layer upon layer; there is also the more specialised Number (up)on Number, to indicate a large quantity, as in hundreds upon hundreds.
N to N can indicate juxtaposition, as in hand to hand, or succession, as in day to day, or transition, as in rags to riches, or comparison.

According to Culicover and Jackendoff (Simpler Syntax, 2005), these violate X-bar theory, in its rule that phrases have heads, and the category of the phrase is determined by that of its head. These NPN constructions often act as if they are adverbs and occur in adverbial positions, they have no apparent head, and almost always appear unmodified. You can occasionally slip in an adjective, as in day by miserable day, but that is unusual and only seldom works.

Ray Jackendoff has written a paper about these: Jackendoff R (2008) Construction and construction and its theoretical challenges. Language 84(1):8-28, which you can download here. I love the way that its final section is entitled “Inconclusion”, which says it all.