Description of problem:

Backport https://issues.jboss.org/browse/JBPM-3866

JBPM-3866 has been addressed by 'group by' at first (commit: e8fbea998621608f02cac6f8963a41a18bb80b36) but now addressed by 'distinct' (commit: 11743329deb101bbb7e26a11f7ae49afa0f24086)


Steps to Reproduce:
1. Create a process with a User Task which has both actorId (="john") and groupId (="PM") 
2. Start a process and let it create a Task
3. call getTasksAssignedAsPotentialOwner with userId and groupIds

        List<String> groupIds = new ArrayList<String>();
        groupIds.add("PM");
        List<TaskSummary> list = localTaskService.getTasksAssignedAsPotentialOwner("john", groupIds,  "en-UK");
        System.out.println("list.size() = " + list.size());

Actual results:

list.size() = 2

Expected results:

list.size() = 1