Hi
Yes you can assign multiple times if you use the Workload redistribution Functionality.
SAp suggested the below reason .. for the same ..which matches your requirement
Documents may have to be manually assigned for the following reasons:
- The person responsible for the given purchasing group is temporarily absent.
- The structure of the purchasing group in your organization has changed.
- The documents fail the automatic assignment rules.
- The rules for the automatic assignment of documents to purchasing groups are not maintained.
If you do not maintain the assignment rule, documents are assigned to a default purchasing group that has to be processed manually.
Refer the links as below as well for your reference :
Workload Distribution - SAP ERP 2005 SAP SRM - SAP Library
http://help.sap.com/saphelp_srm70/helpdata/en/45/f12b3c7e4b60d4e10000000a11466f/content.htm
Regards
Vinita