select ref_item_id from ref_item
where ref_item.REF_TYPE_ID = 9100  and  ref_item.rowid in ( 
select rid
from ( select rowid rid, row_number() over
       (partition by  ref_item.REF_TYPE_ID, ref_item.inv_id
       order by rowid) rn from ref_item  where  ref_item.REF_TYPE_ID = 9100) where rn <> 1 

select ref_item.ref_item_id from ref_item,inv
where ref_item.REF_TYPE_ID = 9100  and  ref_item.inv_id=inv.inv_id and ref_item.rowid in ( 
select rid
from ( select ref_item.rowid rid, row_number() over
       (partition by  ref_item.REF_TYPE_ID, ref_item.inv_id
       order by ref_item.rowid) rn from ref_item,inv  where  ref_item.REF_TYPE_ID = 9100 and inv.inv_id=ref_item.inv_id)
        where rn <> 1 )

Plan
SELECT STATEMENT  CHOOSECost: 595,774  Bytes: 2,584  Cardinality: 76  					
12 NESTED LOOPS  Cost: 595,774  Bytes: 2,584  Cardinality: 76  10 NESTED LOOPS  Cost: 593,097  Bytes: 74,956  Cardinality: 2,677  
8 VIEW SYS.VW_NSO_1 Cost: 405,305  Bytes: 1,311,485  Cardinality: 187,355  						
7 SORT UNIQUE  Bytes: 3,747,100  Cardinality: 187,355  		
6 VIEW R1APP. Cost: 405,305  Bytes: 3,747,100  Cardinality: 187,355  				
5 WINDOW SORT  Cost: 405,305  Bytes: 3,747,100  Cardinality: 187,355  			
4 HASH JOIN  Cost: 404,530  Bytes: 3,747,100  Cardinality: 187,355  		
1 INDEX FAST FULL SCAN UNIQUE R1APP.XPK_INV Cost: 929  Bytes: 26,160,696  Cardinality: 4,360,116  	
3 PARTITION RANGE ALL  Partition #: 9  Partitions accessed #1 - #109	
2 TABLE ACCESS FULL R1APP.REF_ITEM Cost: 393,476  Bytes: 92,481,200  Cardinality: 6,605,800  Partition #: 9  Partitions accessed #1 - #109
9 TABLE ACCESS BY USER ROWID R1APP.REF_ITEM Cost: 1  Bytes: 21  Cardinality: 1  Partition #: 11  			
11 INDEX UNIQUE SCAN UNIQUE R1APP.XPK_INV Cost: 1  Bytes: 6  Cardinality: 1  						

Select from (
select ref_item.ref_item_id ,
row_number() over
       (partition by  ref_item.REF_TYPE_ID, ref_item.inv_id
       order by rowid) rn
from ref_item,inv
where ref_item.REF_TYPE_ID = 9100  and  ref_item.inv_id=inv.inv_id )
where rn>1