The exception occurs when two subsequent tasks are both assigned to the same person and the second task contains a notification node. When the first task is completed, the MailListener will use the findTaskByExecution method of DbSessionImpl.java, which in turn queries the database for tasks by execution. I'm not sure if accidentally two instances of the same task are inserted into the database or if the previous task is not yet removed, but the query apparently returns two results, so the "uniqueResult()" call throws a NonUniqueResultException.

(See attached process and unit test for details, this can also be found in the forum at http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4262559#4262559)