Thanks very much, Chris. That was the way we ended up playing it.
You can deplete any unit you choose whether depleted or not. The only stipulation is that the same unit can not be depleted twice in the same action round. So, for example, if the German player played a combat card on his action round with a 2X depletion effect, both depletion effects could not be used on the same unit. On the other hand, if you deplete a unit on one action round, the same unit could be depleted again on the next action round.