Implement some measures to at least make it harder to use alts to complete tasks
simultaneously. Like, maybe disable kill counts (or reduce rare reward possibility) if game clients connect from the same IP/device, require some activity from alts (constant movement, periodic damage, etc.). Maybe the more often the same accounts perform the same tasks, the less often they receive a rare reward (or a lower percentage of the base reward as well). So it will be necessary to constantly complete different tasks.