While creating unique index on a table , got below error.

SQL> create unique index TEST_IDX on TEST5(EMPNO);
create unique index TEST_IDX on TEST5(EMPNO)
*
ERROR at line 1:
ORA-01452: cannot CREATE UNIQUE INDEX; duplicate keys found

Solution:

Find the duplicate rows in the table and delete them.

Use below script to delete the duplicate rows

REM This is an example SQL*Plus Script to detect duplicate rows from
REM a table.
REM
set echo off
set verify off heading off
undefine t
undefine c
prompt
prompt
prompt Enter name of table with duplicate rows
prompt
accept t prompt 'Table: '
prompt
select 'Table '||upper('&&t') from dual;
describe &&t
prompt
prompt Enter name(s) of column(s) which should be unique. If more than
prompt one column is specified, you MUST separate with commas.
prompt
accept c prompt 'Column(s): '
prompt
select &&c from &&t
where  rowid not in (select min(rowid) from &&t group by &&c)
/

SQL> @dupli.sql


Enter name of table with duplicate rows

Table: TEST5


Table TEST5

 Name                                      Null?    Type
 ----------------------------------------- -------- ---------------------------
 EMPNO                                              NUMBER
 EMPNAME                                            VARCHAR2(29)


Enter name(s) of column(s) which should be unique. If more than
one column is specified, you MUST separate with commas.

Column(s): EMPNO


         1
         4

We can see, for empno=1 and 4 we have duplicate rows. Analyze those rows and delete them remove the duplicates, Once duplicate rows were removed, try to create the index.