Steps to reproduce:
Try to execute the following code snippet:

Task newTask = taskService.newTask();
newTask.setAssignee("Horst");
newTask.setOwner("Horst");
taskService.saveTask(newTask);

Problem:
This leads to a unique index violation:

Unique index or primary key violation: "ACT_UNIQ_AUTH_USER_INDEX_6 ON PUBLIC.ACT_RU_AUTHORIZATION(TYPE_, USER_ID_, RESOURCE_TYPE_, RESOURCE_ID_)"