Etymonline proclaims that 'already' did literally mean 'all ready'.
c. 1300, "in a state of readiness" (an adjectival sense, now obsolete), literally "fully ready, quite prepared," a contraction of all + ready (adj.).
So what semantic notions underlie it and Definition 2 below from OED?
2. South African. Used redundantly, esp. after a word or phrase, for emphasis, or in order to indicate the completed action of a verb.
Readiness and completion are unmistakably distinct notions. E.g. at the starting line marathoners were ALL READY to run, but if there's thunder and the race gets cancelled, they didn't ALREADY run.