i think humans would find any way possible to manipulate this system for monetary reward. If someone could hack a computer a bit to emulate two humans to earn twice as much, then that system has a direct monetary reward.. and development shall begin.
Maybe it has to be proof of human with no computer hardware..
the sequence would have to be something that can only happen once. Like a key to the next step would be in a database that purges itself once accessed once. if someone tried to hack the database they would receive a salted hash. Making them feels stupid because its a lot easier to prove they are human than to solve a salted hash.