The English alphabet has the letters C and K, yet they produce the exact identical sound, what is the reason for this redundancy, and what is the difference in usage among English, French, German, Spanish, Italian, etc. of the letters C and K?

Well C in English doesn't always make the same sound, does it? Call and Ceremony, for example. And I often feel the letter C exists to accomany the H, like in Chalk. Many language have CH as a sepereate letter entitiy, eg Welsh.

And as for the existence of C in German, it is there, but it is only usually used when accompanied by an H, or in the SCH construction. Otherwise, K is used.

As this site explains, the Latin language nearly abandoned the Greek "kappa," preferring "C," which ultimately led to redundancies when Greek and other foreign words that involved a hard "k" sound were adopted and then absorbed into English.

The Romance alphabets include the letter "k" only for loan words. In Italian, the "k" sound is otherwise represented by the combination "ch." For example, the word "chiaro" ("clear") is pronounced with an initial "k" sound -- "kee-AH-ro."

Another good reason why English spelling drives people crazy.

https://en.wikipedia.org/wiki/K

And in German, when it is found without the H, it is usually a soft C, as in cellular, or Celle (a city in northern Germany).

Both C and K are of Semitic origin, and in the Semitic languages they are entirely separate. C has the same root as "Gama" or Gimel, whereas K (as Franco points out) comes from Kappa, or Kaf. Take the sound one step further and you get Quf, or Latin Q, which without the U denotes a deeper click from within the throat. (Arabic has a letter for this, and some Hebrew speakers still pronounce the deep Quf to deferenciate it from the soft Kaf.)

In Spanish, c can be s or th in sound when soft, but it's a k when hard (in front of a, o and u). Spanish doesn't use k (except for things like kilo). Fench c sounds as k when in front of a, o and u, and s in front of e and i - unless it's got a ¸ under it. Ç is always soft in French. French only uses k for borrowed words like kiosk (from Turkish) and kilo (from Greek). Breton uses k - the town known as Quimper in French is Kemper in Breton - so you may find places in France with k in them.