In the first alphabet that we know of, the Phoenician, there is both a 'kaph' letter and a 'qoph'. These are the long-term ancestors of our K and Q. They were pronounced differently in Phoenician (see Qoph ; Kaph was pronounced more like our K) so Qoph was not redundant.

thousand, K (thousand)

k, eleventh letter of the alphabet. It corresponds to the Semitic kaph and the Greek kappa (Κ). It has changed its shape less perhaps than any other letter in the history of the alphabet.

K comes from the Greek word kilo which means a thousand. The Greeks would likewise show million as M, short for Mega. So if we stay consistent with the Greek abbreviations, then billion would be shown as a letter G (Giga). Think of your computer expressing bytes of memory as kilobyte, megabyte, or gigabyte.