Fix a bug introduced with OFBIZ-5761. This appeared only in a custom project when using the underlying services. Then after creating a new ship group with createOrderItemShipGroup, when appending and item with appendOrderItem the OISGAs (OrderItemShipGroupAssoc) where duplicated

git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1643172 13f79535-47bb-0310-9956-ffa450edef68