Can you use cards/abilities to ready cards that are already ready, or exhaust cards that are already exhausted?
Ready and Exhaust
Don't have the RRG handy to give a citation, but no.
It's a little deeper than that. The following is the relevant entry:
RRG, p. 4 - Card Effect: "- A card ability can only be initiated if its effect has some potential to change the game state, and its cost (after modifiers) has the potential to be paid in full.
- A card ability may be initiated if all or part of its effect is able to resolve. Once initiated, players must resolve as much of an effect as they are able, unless the effect uses the word 'may.'"
So, if the only thing a card effect does is ready (or exhaust) a unit, you cannot use it on a unit that is already ready (or exhausted). It's worth noting that such units would not be legal targets of such effects, either.
However, if a card effect both readies a unit and does something else, you might be able to still use the effect if all your units are ready (for the "something else"), depending on how the exact wording of the effect.