In our original implementation, quest item drops were rolled on the
spot, so the chances of getting two quest items for different missions
in a single kill (where both missions have you kill the same mob) were
independent of each other.
When we made quest item drop chances shared between group members so
players doing missions together would progress at the same rate, we
accidentally linked the quest item odds of different missions together.
This change makes it so that the odds are per-task, so they're shared
between different group members doing the same tasks, but distinct for
different tasks being done by the same player.
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