Adding opportunity cost and renting cost isn't double counting.
Opportunity cost is the foregone block rewards that you lose because you didn't submit your blocks, because you were holding them hoping to build a long enough chain to double spend. When you fail, that reward that you would have earned is gone forever.
Renting cost is the actual $ outlay that it costs you to rent the hash power necessary to perform the attack.
Opportunity cost is the foregone block rewards that you lose because you didn't submit your blocks, because you were holding them hoping to build a long enough chain to double spend. When you fail, that reward that you would have earned is gone forever.
Renting cost is the actual $ outlay that it costs you to rent the hash power necessary to perform the attack.