This bug comes from http://forums.activiti.org/en/viewtopic.php?f=6&t=2218. More information could be found there.
create a timer start event, set the time to a simple time duration, something like
and connect it to a simple service task. It is expected that the service task triggers once after the specified time.
But it is triggered twice. One at the right moment and one just when the process is started.
A test case showing the bug is attached.