hunker down. does it work on a ready unit already?

Yes — it's a legal play on an already-ready unit, but the "Ready" portion does nothing.

Rule 415.1.c says if a unit is instructed to be Readied while it's already Ready, nothing additional happens (415.1.b: a Ready unit can't be Readied again). It's still a legal choice, same way an already-buffed unit can be chosen for a Buff effect — you just don't get that part.

The rest of Hunker Down still resolves on the chosen unit: it can't move this turn, and you draw 1. So the spell is never a "whiff," it just becomes a 1-energy draw with a movement lock if you point it at something already upright.