Non Unique Index Using A Non Unique Field

Using Oracle 11g, below is the table, partitions, unique and non-unique local index: 

CREATE TABLE DOCA(  DOCA_ID  NUMBER  NOT NULL ,  DOCA_BKG_PAX_ID  NUMBER  NULL ,  ROW_PURGE_DATE  DATE  NULL ,)PARTITION BY RANGE(ROW_PURGE_DATE)INTERVAL(NUMTOYMINTERVAL(1, 'MONTH'))(  PARTITION P2007 VALUES LESS THAN (TO_DATE('01/01/2008', 'dd/mm/yyyy')),  PARTITION P200801 VALUES LESS THAN (TO_DATE('01/02/2008', 'dd/mm/yyyy')),)  TABLESPACE T0; ALTER TABLE DOCA ENABLE ROW MOVEMENT;    

CREATE UNIQUE INDEX XPKDOCA ON DOCA(  DOCA_ID  ASC,  ROW_PURGE_DATE ASC)LOCALREVERSE TABLESPACE I0;  ALTER TABLE DOCA  ADD CONSTRAINT  XPKDOCA PRIMARY KEY (DOCA_ID);    CREATE INDEX XFKDOCA_DOCA_BKG_PAX_ID ON DOCA(  DOCA_BKG_PAX_ID  ASC)LOCALREVERSETABLESPACE I0;  

I would like to know the difference between the performance of the unique and non-unique local indexes?.