This issue looks like Bug: 13729601
The dequeuer returns the same message in multiple threads in high concurrency environments when Oracle database 11.2 is used. This means that some messages are dequeued more than once. For example, in Oracle SOA Suite, if Service 1 suddenly raises a large number of business events that are subscribed to by Service 2, duplicate instances of Service 2 triggered by the same event may be seen in an intermittent fashion. The same behavior is not observed with a 10.2.0.5 database or in an 11.2 database with event10852 level 16384 set to disable the 11.2 dequeue optimizations.
Workaround: Perform the following steps:
Log in to the 11.2 database:
CONNECT /AS SYSDBA
Specify the following SQL command in SQL*Plus to disable the 11.2 dequeue optimizations:
SQL> alter system set event='10852 trace name context forever,
Restart the database.
Edited by: Maarten Smeets on Jan 28, 2013 2:10 PM