Update table from different table
Quick and dirty answer:

UPDATE RBI.RBI_OV_F_DETAIL A
 SET (BRANCH_ID, BRANCH_NUMBER) = (
       SELECT PRICE_BRANCH_ID, PRICE_BRANCH
         FROM RSE.RSE_SO_F_ORDERS B
            WHERE A.SOURCE_ID = B.FULL_ORDER_NUMBER AND A.METRIC_GROUP_ID='1'
       group by price_branch_id, price_branch);


However if you're not expecting duplicates in the RSE.RSE_SO_F_ORDERS table, the more accurate answer would be to hunt down why those duplicates are there, and work with any stakeholders to eliminate the duplicates.