Though it is traditionally called "Second Clement", implying that the same author is responsible for both First and Second Clement, scholarship has for centuries (as I recall) considered them to come from different sources. Lightfoot, in his one-volume edition from the late 19th century, simply calls it "An Ancient Homily" instead of "Second Clement".

As I've been examining instances of αλλα in the Apostolic Fathers, I ran across a peculiar thing.

In First Clement (and in NT, for that matter), when the phrase "not only ... but (also) ... " is used, the phrasing is "ου μονον .. αλλα και" with uniformity (though cf. 2Ti 2.20, Εν μεγαλη δε οικια ουκ εστιν μονον .. αλλα και ..).

In Second Clement, however, the phrasing of "not only ... but (also)" is uniformly "μη μονον .. αλλα και" (or some variant of μη μονον, like 2Cl 9.10, "μη απο στοματος μονον αλλα και .. " or even 2Cl 13.1, "και μη .. μηδε θελομεν μονον .. αλλα και ..").

I'm not one to say that an author always has to use the same turn of phrase in the same way. But the disparity between First and Second Clement in this sort of phrasing seems suspicious.

Update (2008-03-19): Note that μη μονον is used elsewhere in the corpus of the Apostolic Fathers: IgnMag 4.1; IgnRom 3.2 (2x); MPoly 1.2 (2x); EpDiog 2.1.