http://scn.sap.com/thread/3383394
Hi Kusuma,
Kindly define Check FMs in SWE2 to check whether any active instance of the workflow exists for the same Sales Order. If an active instance for the Custom Workflow Template exists for the same key(Sales Order#), then raise the exception in the check FM.
The exception will ensure that another instance of the workflow does not trigger for the same Sales order.
Kindly refer to the above link for further details on Check FM concept.
Thanks and Regards,
Vikram