I don't remember ever hearing "competency" until circa 1970. When I first heard that form it sounded very silly, as if someone was trying to sound lofty via the addition of superfluous syllables.
Is there a context where "competency" is correct usage, and "competence" wouldn't do just as well?
Is it a recent (last 40 yrs) permutation, or is it my sheltered background that made it sound that way?
(Is anyone here old enough to know?)
Okay, that's three questions, sorry!