Since I can't find my source for this information, I formulate this question in two parts:
If a baker is shlashed twice in the same cycle for double (pre-)endorsing he has no frozen_deposit. Does this result in him losing his baking/endorsing rights for the remainder of the cycle?
If 1. applies, is the quorum for block endorsement lowered? What happens when 34% of the total staking power double (pre-)endorses twice and loses its baking rights?