HibernateException: Found two representations of same collection

The problem was, that Hibernate does not support the combination of @OneToMany(mappedBy=...) and @OrderColumn. Without mappedBy Hibernate uses a join table and everything works as expected. See explanation.

The same error occurs when you try to modify an associated collection of an object. e.g.:

    MyObject myObject = myObjectService.get(id);
    List<Task> newTasks = //populate new list of Task here
    myObject.setTasks(newTasks);
    myObjectService.saveOrUpdateObject(myObject); // or merge(myObject)      

In such a case, it can be resolved by:

    MyObject myObject = myObjectService.get(id);
    List<Task> newTasks = //populate new list of Task here
    myObject.setTasks(new List<Task>); // or myObject.getTasks().clear();
    myObject.getTasks().addAll(newTasks);
    myObjectService.merge(myObject); 

I could resolve the issue by changing the association to lazy and removing the cascade.

@OneToMany(mappedBy="taskList", fetch = FetchType.LAZY)
@OrderColumn(name="position", nullable=false)
public List<Task> tasks = new ArrayList<Task>();