Of Course when the Constraint is disable it will check the validity whenever it will be disabled and it will not prevent the Future dupicate rows.

Better to remove the dupicate rows, enable the Contraint and it will not have any duplicate rows in future.

Duplicate Selection?
Select * from table1 a
where rowid not in (select max(rowid) from table1 b
where a.col1=b.col1)

Table1 is Your Table... in Subquery all the columns that will be checked for duplication...

To Delete, suppose from emp table I want to remove dupicate empno...
Delete emp a
where rowid not in (select max(rowid) from emp b
where a.empno = b.empno);

Thanks