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delete from scott.emp where empno in
(
select empno from scott.emp
group by empno
having (count(empno)>1)
)

but the query will delete all duplicate entries in a table with out keeping one copy

As per my knowledge,There is no option to delete duplicate rows with out rowid

Source: http://www.oraclepassport.com/forums/index.php/topic,392.0.html