If the G clef ends up overriding the C Clef, why include the C clef in the first place? Questionable image quality, but I couldn't find a legible example. I'm aware that it's a dumb question, but I can't help but wonder. The sample image is from the very first exercise in First Species counterpoint, but the same thing occurs throughout the whole book.
That's a so-called "incipit" indicating the original clef that a piece has been written in as opposed to the publication you now have in hand. Fux lived in 17th and 18th century, so the clefs used in his life time were different from what one would now consider appropriate.
The incipit usually also contains the original key signature (apparently the original key was C major as well, so there is not a lot to be seen) since often a current version is also transposed from the original notation (which might have assumed a different tuning anyway).
It's not unusual to also add a time signature since modern notation tends to prefer working with shorter nominal durations than older music.
So in this particular case, the "incipit" is rather minimal, but I am pretty sure that this is what it is.