[coolibin]

Кхм...попробовал, у меня на трешке не выдает дублей никаких, и даже без changed()

    qbDSItem = query.addDataSource(tableNum(InventTable));
    qbDSItem.addSortField(fieldNum(InventTable, ItemId));
    qbDSItem.orderMode(OrderMode::GroupBy);
    qbDSItem.addSelectionField(fieldNum(InventTable, ItemId));
    //добавляю таблицу InventTrans
    qbDS = qbDSItem.addDataSource(tableNum(InventTrans));
    qbDS.joinMode(JoinMode::OuterJoin);
    qbDS.addLink(fieldNum(InventTable, ItemId), fieldNum(InventTrans, ItemId));
    qbDS.addSortField(fieldNum(InventTrans, ItemId));
    qbDS.orderMode(OrderMode::GroupBy);
    //вывожу результат
    queryRun = new QueryRun(query);
    while (queryRun.next())
    {
        inventTable = queryRun.get(tableNum(InventTable));
        inventTrans = queryRun.get(tableNum(InventTrans));
        //if (queryRun.changed(tablenum(InventTable)))
            info( strfmt( "%1 %2", inventTable.ItemId, inventTrans.ItemId));
    }

добавил qbDS.orderMode(OrderMode::GroupBy);