Perhaps this is the intended behavior, but calling taskService.addTaskParticipatingUser with the same user creates multiple participates:

taskService.addTaskParticipatingUser(_task.getId(), "boss_rigging", Participation.CANDIDATE);
taskService.addTaskParticipatingUser(_task.getId(), "boss_rigging", Participation.CANDIDATE);

assertEquals(1,taskService.createTaskQuery().candidate("boss_rigging").activityName("Investigate").list().size());

The query comes back with two tasks. This is somewhat surprising.